AM Tuner -- Build/Find a high performance modern tube AM Tuner ?
Craig Bridge
To add to the collection of modern tube components for my audio system, I am
looking for a high-performance, "modern design" tube-based AM tuner (mono,
not AM stereo.) I'd either build it myself from a schematic, build it from a
kit (if one exists meeting my reqs), or simply find a pre-built one (again,
if one exists meeting my reqs.)
A search on the Internet came up with very little other than the following
web page by Bruce Carter, "Building Your Own AM Tuner":
http://www.mindspring.com/~brucec/amtuner.htm
I also checked with the "World Tube Audio Portal", and could not find even a
single mention of tube tuners/receivers:
http://www.worldtubeaudio.com/
(Understandably, tube audiophiles are mostly interested in playback of CD
and vinyl, and so are focused on pre-amps, phono-amps and amps.)
Here's the general requirements for my "dream" AM Tuner:
1) Modern tube design. Uses easy-to-find tubes likely to be manufactured
into the future.
2) Tuner only, not a radio. It would have a line-out of the proper voltage
and equalization to connect to a contemporary pre-amp/amp. It would become a
component in a modern audio system.
3) Good sensitivity, selectivity, dynamic range, etc., adequate for most MW
DXers (except maybe those who wish to pull in 1 watt stations from Outer
Bulgravia.) Probably would have a variable bandwidth control, among other
controls to aid in DXing.
4) Excellent line-out audio quality for stronger and local stations. (Again,
variable bandwidth will help here.)
5) Uses as many parts as possible which are manufactured today. (Concern
about components such as tuning capacitors----are there any components which
are not available "off-the-shelf"----antique radios do not count, or would
cost a small fortune to have built on spec?)
6) Tunes in the current MW band (510 to 1705 kHz) and maybe a little outside
of this range just in case the FCC further expands it.
Obviously, one approach the antique radio collectors will suggest is to find
an old 1930's to 1960's tube radio/tuner and simply restore it. Well, this
will be considered, but such radio will be totally restored with almost all
the components replaced, rewired onto a new modern looking chassis, etc.,
etc. (It's like a hotrodder buying a Model T Ford and replacing every part,
except the body which itself will be dramatically changed, with all modern
equipment -- is it a Model T anymore?) I'd hate to "destroy" a classic radio
doing this, which is what will happen. I'd prefer to simply replicate (with
tweaks as needed to meet the reqs) the electronics of a highly touted radio
design (suggestions?). Anyway, the requirement for a tuner with a line-out
of the proper voltage and equalization might make this pathway slightly more
complicated----I leave it to the radio experts here to clarify this.
As an aside, I think it would be grand for an Internet project to start
which would select, adapt and "modernize" a classic tube radio design per a
list of requirements similar to those listed above! Issue the schematic,
parts list and suggested vendors, etc.
Thanks in advance for your thoughts on this request.
Craig
Cardinal Class of '67
John Walton
receiver with a loop aerial -- methinks that the loop antenna has a lot to
do with the excellent sensitivity.
If you don't go this route, the most difficult item to find for a super-het
will be the ganged 365uF (they called 'em 365mmF) capacitor and the IF
transformers.
Jack
"Craig Bridge" <crai...@hotmail.com> wrote in message
news:b2a409b34038cb3c...@free.teranews.com...
Robert Casey
These requirements tend to conflict. For hi-Fi work, you'd want a wide
passband of
20KHz (believe it or not, AM stations are allowed to transmit audio of
up to 10KHz
thus meaning that the spectral bandwidth is 20KHz). Stations are spaced
at 10KHz
intervals, but not in the same city. A simple TRF (tuned radio
frequency) tuner with
an infinate impedance detector would supply hi-fi reception of local
stations. But no
DX. But for DX you'd want a narrower band to reject adjacent stations.
I built a
radio with an automatic variable bandpass feature controlled by the
signal strength
via AVC. Wide passband for "HiFi" reception of strong locals, and
narrow for the
out of town weak DX. See:
http://www.geocities.com/wa2ise/radios/12ba7.html and more specifically
http://www.geocities.com/wa2ise/radios/plateresistance.jpg
Manual variable passband could be done using pots as reostats connected
across
the secondaries of IF transformers, 1 meg is about the right value to
use, and
physically locate the body of the pot near the IF transformer (to avoid
wires
carrying IF signals causing stray oscillations from stray couplings).
For the infinite impedance detectors:
http://home.netcom.com/~wa2ise/radios/AA5submini.html#infdet
>
>4) Excellent line-out audio quality for stronger and local stations. (Again,
>variable bandwidth will help here.)
>
>5) Uses as many parts as possible which are manufactured today. (Concern
>about components such as tuning capacitors----are there any components which
>are not available "off-the-shelf"----antique radios do not count, or would
>cost a small fortune to have built on spec?)
>
>
>6) Tunes in the current MW band (510 to 1705 kHz) and maybe a little outside
>of this range just in case the FCC further expands it.
>
>
>Obviously, one approach the antique radio collectors will suggest is to find
>an old 1930's to 1960's tube radio/tuner and simply restore it. Well, this
>will be considered, but such radio will be totally restored with almost all
>the components replaced, rewired onto a new modern looking chassis, etc.,
>etc. (It's like a hotrodder buying a Model T Ford and replacing every part,
>except the body which itself will be dramatically changed, with all modern
>equipment -- is it a Model T anymore?) I'd hate to "destroy" a classic radio
>doing this, which is what will happen. I'd prefer to simply replicate (with
>tweaks as needed to meet the reqs) the electronics of a highly touted radio
>design (suggestions?). Anyway, the requirement for a tuner with a line-out
>of the proper voltage and equalization might make this pathway slightly more
>complicated----I leave it to the radio experts here to clarify this.
>
An option is to look for an old radio with a damaged cabinet. Or one
with the
shape of cabinet that would match the rest of your system. Most AM
radios are the "hot chassis"
type (AA5), but you could remove the audio and power supply circuits,
and keep and restore
the tuner part. New construction of RF and IF stages can be tricky to
avoid self oscillation, so
making use of an existing chassis or circuit board would be a wise
approach. The circuits will need
around 95 volts B+ (not critical) at around 20ma or less, and of course
heater current. You can
swap the 12 volt tubes for their 6 volt versions easily if need be. I'd
select a post WW2
set; the parts are fairly easy to get as most AM radios used the same
design.
No equalization is really needed, unlike FM. Some stations may boost
their treble to
compensate for narrow AM radio passband a little.
>
>
>
tedpeterson8
AES's "Hi-Fi Tuner Kit", they claim a crystal radio for AM reception delivers
the upmost in fidelity. Part # K-420 http://www.tubesandmore.com
Thal70
Doubt you'll ever find anything new that would come close to your
request. No question your best bet is to find a tube tuner from the 50's or
60's as you mentioned. There are "tons" of them. Find a good model in
excellent shape and have it restored. Some don't need much restoration. Few
if any will meet all of your demands but I doubt you would be disappointed.
I have quite a few tube tuners -- most are FM as well but have many features
with excellent sensitivity. Research HH Scott, Fisher and even Grommes.
Here is a thread from another site which may help:
http://www.audioasylum.com/forums/vintage/messages/11269.html
"Craig Bridge" <crai...@hotmail.com> wrote in message
news:b2a409b34038cb3c...@free.teranews.com...
Robert Casey
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3034424979&category=296
(not mine).
don
use tubes such as 12at7. Are any being currently produced?
John Stewart
Should show up on Ebay now & then.
Good Luck, John Stewart
Patrick Turner
Craig Bridge wrote:
> Everyone,
>
> To add to the collection of modern tube components for my audio system, I am
> looking for a high-performance, "modern design" tube-based AM tuner (mono,
> not AM stereo.) I'd either build it myself from a schematic, build it from a
> kit (if one exists meeting my reqs), or simply find a pre-built one (again,
> if one exists meeting my reqs.)
>
> A search on the Internet came up with very little other than the following
> web page by Bruce Carter, "Building Your Own AM Tuner":
>
> http://www.mindspring.com/~brucec/amtuner.htm
>
> I also checked with the "World Tube Audio Portal", and could not find even a
> single mention of tube tuners/receivers:
>
> http://www.worldtubeaudio.com/
>
> (Understandably, tube audiophiles are mostly interested in playback of CD
> and vinyl, and so are focused on pre-amps, phono-amps and amps.)
>
> Here's the general requirements for my "dream" AM Tuner:
>
> 1) Modern tube design. Uses easy-to-find tubes likely to be manufactured
> into the future.
There are very few vari-mu RF pentodes made these days, if any, and afaik,
absolutely no frequency converters such as 6BE6.
So best you can do is use the most common tubes made up to about 1970.
A reasonable AM tuner can be done with a double tuned
input RF circuit, then a 6BE6 mixer, followed by an 455 KHz IFT with a tertiary
winding which
can be switched in to increase the coupling. IFTs are no longer made, so old
ones have to be addapted.
The next tube should be 6BA6, with a following IFT.
The IF output should go to a 1/2 a 12AU7 triode cathode follower, to power a
germanium diode which has a ma of bias current running in it, so that the diode
non linearities are nearly eliminated,
so that even on low signal levels, the detector distortion is less than 1%.
After RF filtering, the second 1/2 of the AU7 can be a gain tube, or a CF
buffer.
An arrangement with another germanium diode to generate a -ve going voltage from
the detector
has to be designed to provide an AVC bias voltage to be applied partially to the
mixer, and
wholly to the 6BA6 IF amp, to keep stations sounding about the same level.
Such a tuner should be OK for about 9 kHz of AF BW.
The switched tertiary winding increase the IF bandwidth from about 5 kHz,
about right for most IFTs, to about 8.5 kHz, and some RC compensation
to emphasize the recovered audio can boost the BW a little further.
Such an AM tuner is great on local stations, and not too bad on DX
if you have a large long wire antenna.
I never listen to DX, because of the whistles, noise, and monkey chatter, and
fade out.
If an RF amp is added with another 6BA6 stage, the problem with DX
is still not much better.
>
> 2) Tuner only, not a radio. It would have a line-out of the proper voltage
> and equalization to connect to a contemporary pre-amp/amp. It would become a
> component in a modern audio system.
See above.
Maybe get the parts from a tubed receiver from 1960,
and transfer them over to a purpose built chassis.
>
> 3) Good sensitivity,
Easy with any AM radio...
> selectivity,
Not so easy if TRF design is used, except for local stations spread apart by say
100 kHz.
> dynamic range,
Easy with tubes.
> etc., adequate for most MW
> DXers (except maybe those who wish to pull in 1 watt stations from Outer
> Bulgravia.)
You won't get much from over 500 Kms away, unless conditions are freakish.
> Probably would have a variable bandwidth control, among other
> controls to aid in DXing.
Hence a tertiary winding on the IFT.
Another way is to have variable distance between coils on the IFT.
and another way is to use an extra IFT, and amp tube, the
IF tubes don't have to be high gain, so old octal pentodes are fine, 6U7, etc,
RF BW is less, and so is AF.
>
>
> 4) Excellent line-out audio quality for stronger and local stations. (Again,
> variable bandwidth will help here.)
See above to get low thd.
In my superhet radio, I have a twin triode vari-mu RF amp,
following a pair of stagger tuned input coils, then a 6AN7 mixer,
then 6BX6 sharp cut off IF amp, variable coil distance on IFTno1.
The CF detector and buffer follows IFTno2.
AVC is only applied to the twin triode RF amp.
Thus the IF amp creates little thd when it amplifies the IF envelope
prior to detection.
6BA6 has more thd.
>
>
> 5) Uses as many parts as possible which are manufactured today.
Impossible to do that with tubes.
Its easy with fets, ceramic filters, balanced fet mixers, small coils, etc....
discrete component AM radio is better than most 3 pin chips used for complete
AM radio detection, all in a package the size of one transistor.
> (Concern
> about components such as tuning capacitors----are there any components which
> are not available "off-the-shelf"----antique radios do not count, or would
> cost a small fortune to have built on spec?)
I wish you well finding suitable varicaps.
So maybe you'll have to use variable inductance coils, and fixed caps.
>
>
> 6) Tunes in the current MW band (510 to 1705 kHz) and maybe a little outside
> of this range just in case the FCC further expands it.
Don't hold your breath waiting for the band to be expanded.
>
>
> Obviously, one approach the antique radio collectors will suggest is to find
> an old 1930's to 1960's tube radio/tuner and simply restore it. Well, this
> will be considered, but such radio will be totally restored with almost all
> the components replaced, rewired onto a new modern looking chassis, etc.,
> etc. (It's like a hotrodder buying a Model T Ford and replacing every part,
> except the body which itself will be dramatically changed, with all modern
> equipment -- is it a Model T anymore?) I'd hate to "destroy" a classic radio
> doing this, which is what will happen. I'd prefer to simply replicate (with
> tweaks as needed to meet the reqs) the electronics of a highly touted radio
> design (suggestions?). Anyway, the requirement for a tuner with a line-out
> of the proper voltage and equalization might make this pathway slightly more
> complicated----I leave it to the radio experts here to clarify this.
But quite a few good AM tuner parts reside in old japanese made tube receivers,
such as Kenwood AM/FM, which had two AM tuners within,
so that one channel of FM could be recieved, and the other channel was AM.
All this happened before the Zenith system was addopted.
>
>
> As an aside, I think it would be grand for an Internet project to start
> which would select, adapt and "modernize" a classic tube radio design per a
> list of requirements similar to those listed above! Issue the schematic,
> parts list and suggested vendors, etc.
Almost no demand for good AM receivers these days.
Just a few diehard old gizas want such stuff.
But, if I were going to make another tube AM tuner,
I'd think about a similar front end as described,
then a mixer, then an IF frequency of 2.5 MHz,,
which will give broader RF BW, with automatically wider AF BW,
and have at least 3 IFTs.
I also tried an all tube direct conversion synchrodyne,
but that was a flop for many reasons, and I see why it never became popular.
It took me a continuous full time month to build a complete
superhet radio from the bits I had accumulated in the shed.
I wound the RF coils, and made copper boxes for them.
This all taught me how capricious and difficult a simple radio is to build.
But my radio has 10 kHz of AF BW, very low thd, and sounds far better than
anything
else I have used.
I have a Quad AM tuner, with tubes, and it isn't quite as good,
but it is good, and it has SW. Now that Quad is a nice bit of gear!.
Patrick Turner.
Steve
they can sound great when hooked up to an amp. If you're in an
urban area, you'll have to put more effort into the quality of the
L-C tuning network for better selectivity.
Steve W6SSP
p.s.-you can always add a 12AU7 preamp if you must have a tube ;-)
Yves Monmagnon
> I also tried an all tube direct conversion synchrodyne,
> but that was a flop for many reasons, and I see why it never became
popular.
>
Could you tell a bit more about that ?
Yves.
John Byrns
<jdwa...@comcast.net> wrote:
> Pick up the most recent issue of AudioXpress -- a nice easy to build TRF
> receiver with a loop aerial -- methinks that the loop antenna has a lot to
> do with the excellent sensitivity.
What's the "most recent" issue of AudioXpress? I'm not a subscriber, but
Tower Records has the July issue, so I assume the August issue is the
"most recent"? Does this easy to build TRF receiver use a tube or
transistor circuit?
> If you don't go this route, the most difficult item to find for a super-het
> will be the ganged 365uF (they called 'em 365mmF) capacitor and the IF
> transformers.
Doesn't a TRF receiver also require a "ganged 365uF capacitor"?
Regards,
John Byrns
Surf my web pages at, http://users.rcn.com/jbyrns/
Patrick Turner
Yves Monmagnon wrote:
Crikey, it'd take a bloody book.
The first real proper synchrodyne afaik was D.G.Tucker's circuit, in
1947,
with an RF amp using two pentodes with NFB and inptut tuning circuit,
then a locked oscillator,
third pentode, then a balanced detector using a toroidal transformer
and 4 diodes. I never got the detector coils to work, and the RF amp
teneded to be be very unstable, and the locked oscillator tended to be
modulated
by the synchronising injection from the input amp, and
all in all, it seemed like a real PITA.
Then when I eventually got a sychrodyne to work with a self oscillating
6BE6,
the output was tiny, it was unstable, and the supposed selectivity due to
AF filtering
still left plenty of weird tones and crap in the AF output.
Commercially viable synchronous detection had to wait until the days of
chips and PLL,
and then it works OK.
And tuning a synchro means lots of whistles and howls until the set locks
on a station.
so more tubes are needed for a complex muting circuit.
In this country, there were never any built commercially, afaik.
Ppl stuck to plain old 5 valve superhets.
Then eventually, after an absurdly long wait due to government bungles
and
vested interests in radio, we got FM in 1972, and the tube era was over
then.
The RDH4 has some FM circuits, which very few ppl could understand, let
alone build.
And very few know how multplex decoders work.
Then there was tubed TV, and that was really baffling.
But many radio amateurs at least made their own receivers, and there were
kits you could buy,
but none of the parts used in 1960 are still available except as pulls
from other gear.
There are mountains of old but not very collectable valve radios about.
something made about 1960 will have fairly miniturised parts,
and have at least a two gang tuning cap.
At the end of the day, what use is an AM radio? All the commercial
stations here are talkback amounst blue collar uneducated folks, so
its the blind leading the blind, or it pop music, usually from a small
selection of records.
So basically, commercial radio is complete crap.
But we at least have the print handicapped radio station, where they read
the papers each day.
Then we have the ABC, the govt rum station, and the afternoon talkback
is amoungst those with degrees fro a uni, and its actually interesting to
listen in.
The music shows on the ONE station and public interest interviews demand
a decent
AM reciever, so I built one from scratch, because the usual
5 valve superhet has so many inherent problems of small AF bandwidth,
and high thd/imd. These problems can easily
be addressed, and AM can nearly be as good as FM to listen to.
The transmitted signal is usually subject to a filter at 9 kHz,
and since OZ stations are all 9 kHz apart, with DX,
one gets 9 kHz whistles, with chatter each side due to the
beating of the station you are on with sidebands of a nearby station.
A 9 kHz notch filter only removes the 9 kHz whistle, and not the
other whistly chatter.
To avoid this interference AF bandwidth has to be limited to about 4 kHz,
and this is the usual AF BW of most tube radios.
Many only have 2 kHz.
2 Khz is typical of many SS radios, because the IFT are not transformers
but single
winding coils, so the shape of the selectivity curve is lousy.
at leat 5 tuned circuits are needed for a radio, and
4 of the 5 shoulod be those in an IF amp, where the critical
coupling of the IFTs result in a flat topped bandpass filter.
Three IFTs are good, but each IFT further reduces AF bandwidth,
so to get more AF BW with steep sided bandpass filter action, a higher
frequency
has to be used for the IF, hence the idea of 2.5 MHz.
So that if the staion recieved is at 500 kHz, the oscillator is at 2 Mhz,
just above the top station frequency. at a staion of 1,600 Khz,
the oscillator is at 3.6 Mhz. The oscillator F range is easily tuned.
The coils for IFT at 2.5 MHz might be easier to wind than 455 kHz,
and they could be wound on existing 455 kHz formers.
They need less turns, and less capacitance.
With an IF of 2.5 MHz, the filtering of RF out of the recovered audio
would be very easy.
The pentodes used for IFamps will work just as well.
With two IF amps, only the first one really needs to be subject to AVC
for MW locals.
But for short wave, obviously, the 2.5 mHz IF would restrict you to
stations above
3 Mhz.
Once one considers SW, up to say 30 Mhz, one has to think about
quitet inputs, quiet mixers, maybe doble conversion perhaps mechanical
or crystal filters, then there is the BFO, for sideband reception, and so
on.
A good SW radio will pick up a few radio hams on the bands.
But for SW, antennas become important, and for the amateur bands,
rotating beam antennas are the go, so that someone using a single
807 in a transmitter in japan, can be heard OK in Canberra.
The Racal communications radio was one of the best ever made,
with a cast and machined aluminium chassis, to seperate all the stages,
and this had 22 valves.
Digital techniques dominate HF radio now, and receivers just digitise the
antenna signal,
whatever it is, and get a computer to count what is in there, and just
extract certain numbers,
and there is your siganl, at any requency selected, its more like a noise
analyser.
I know hams who built everything in the shed, all the test gear,
and all the transmitters and modulators, etc.
They are mostly retired, and getting old, and they craft will die with
them
because HF radio is becoming an old fashioned way to communicate
long distance.
And that communication is subject to ionsphere condtions in 11 year
cycles,
and lots and lots of bloody noise and interference.
Then there are all the problems of restrictions on radio gear in built up
areas.
I doubt I will ever become a ham radio guy.
I find too much activity at AF frequencies, which prevents me going
higher.
Patrick Turner.
Patrick Turner
John Byrns wrote:
For TRF, 4 single tuned circuits are required, so that means two
double gang tuning caps, and then you have to get them all to track properly.
The Q of each tuning circuit dosn't want to be too high, or else the
final BW will be so narrow that so will the AF BW.
They invented the superhet to overcome the TRF troubles.
Patrick Turner.
Fred Nachbaur
Patrick Turner wrote:
>
> John Byrns wrote:
>
>
>>In article <G3OdnRd7ovd...@comcast.com>, "John Walton"
>><jdwa...@comcast.net> wrote:
>>
>>
>>>Pick up the most recent issue of AudioXpress -- a nice easy to build TRF
>>>receiver with a loop aerial -- methinks that the loop antenna has a lot to
>>>do with the excellent sensitivity.
>>
>>What's the "most recent" issue of AudioXpress? I'm not a subscriber, but
>>Tower Records has the July issue, so I assume the August issue is the
>>"most recent"? Does this easy to build TRF receiver use a tube or
>>transistor circuit?
>>
>>
>>>If you don't go this route, the most difficult item to find for a super-het
>>>will be the ganged 365uF (they called 'em 365mmF) capacitor and the IF
>>>transformers.
>>
>>Doesn't a TRF receiver also require a "ganged 365uF capacitor"?
>
>
> For TRF, 4 single tuned circuits are required, so that means two
> double gang tuning caps, and then you have to get them all to track properly.
> The Q of each tuning circuit dosn't want to be too high, or else the
> final BW will be so narrow that so will the AF BW.
The bigger problem is that the Q changes across the band, tending to be
higher at the top end because of the higher reactance (and therefore
higher ratio of reactance to resistance). So there'a compromise between
good sound but lousy selectivity at the low end, and good selectivity
but muddy sound at the high end. The AM broadcast band is bad that way,
since it's about a 3:1 frequency range.
> They invented the superhet to overcome the TRF troubles.
Indeed. Only one frequency to worry about, as far as the selectivity vs.
bandwidth situation is concerned. Also makes it practical to do
stagger-tuning to give a more square-shouldered bandpass characteristic.
Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+
John Byrns
<fnac...@netscape.net> wrote:
> Patrick Turner wrote:
>
> > John Byrns wrote:
> >
> >>In article <G3OdnRd7ovd...@comcast.com>, "John Walton"
> >><jdwa...@comcast.net> wrote:
> >>
> >>>If you don't go this route, the most difficult item to find for a super-het
> >>>will be the ganged 365uF (they called 'em 365mmF) capacitor and the IF
> >>>transformers.
> >>
> >>Doesn't a TRF receiver also require a "ganged 365uF capacitor"?
> >
> >
> > For TRF, 4 single tuned circuits are required, so that means two
> > double gang tuning caps, and then you have to get them all to track
> > properly. The Q of each tuning circuit dosn't want to be too high, or else
> > the final BW will be so narrow that so will the AF BW.
>
> The bigger problem is that the Q changes across the band, tending to be
> higher at the top end because of the higher reactance (and therefore
> higher ratio of reactance to resistance). So there'a compromise between
> good sound but lousy selectivity at the low end, and good selectivity
> but muddy sound at the high end. The AM broadcast band is bad that way,
> since it's about a 3:1 frequency range.
When you speak of the "low end", and the "high end", are you talking about
wavelength, or frequency?
Actually you want the Q to vary across the band, you want the Q to be
proportional to frequency, so that you can maintain a constant bandwidth
as you tune from low frequencies to high frequencies. This effect can be
approximated by making as many as possible of the lossy elements in the
tuned circuits, series elements. This means the tuned circuit should be
driven from a pentode with a high output resistance, and care should be
taken in the design and layout of the amplifiers to make the grid
conductance as low as possible. series resistors can then be added to the
tuned circuits to set the desired circuit Q. Double tuned circuits, using
two tuning condenser gangs for each stage, can also be used to improve the
filter shape factor and achieve steeper skirts without excessive narrowing
of the nose response. Complex coupling impedances can also be used to
maintain the desired coupling as you tune across the band, and also to
provide increased coupling at the low frequency end of the band to help
offset the effects of the remaining parallel losses in the circuit. These
techniques can greatly reduce the compromises necessary in TRF AM
broadcast receivers. The famous Western Electric No. 10A Radio Receiver
is an example of this type of design. J.W Miller also offers a series of
radios and components based on this design technique, and the J.W. Miller
design was used in Altec's early AM tuners.
> > They invented the superhet to overcome the TRF troubles.
>
> Indeed. Only one frequency to worry about, as far as the selectivity vs.
> bandwidth situation is concerned. Also makes it practical to do
> stagger-tuning to give a more square-shouldered bandpass characteristic.
Why even bother to stagger tune? Double tuned IF transformers, and even
higher order filters, like the fourth order filters used in some of the
H.H. Scott AM tuners, can accomplish much the same thing by judicious
choice of Qs and coupling coefficients.
Robert Casey
flamethrower,
a simple LC circuit with a long wire antenna and a diode detector feeding
a high impedance load (like the input of a tube amp) can sound quite good.
An LC circuit stolen from a transistor portable radio will do fine. You'll
need to connect its ground to a good earth ground like a water pipe, and
couple a "longwire" wire antenna thru a cap of around 20pF to the high
side of the LC circuit. A diode like a 6AL5 (connect its plate to the
high side of the LC circuit, cathode feeds a 50pF cap to ground and also
feeds to a 1 meg load (like the pot inside a tube amp, must be no coupling
caps in the way, though)) should work nicely.
6AL5
longwire----------||----+---+-----|[__________
~20pF e | |
|
e =
= pot-------
e |
| |
earth ground-------------+----+ -----------+---------+------
inductor var cap 50pF
inside your tube amp
The longwire is any convient length around twenty to forty feet, away from
power lines.
John Stewart
The Eico HFT-94 has most of the features you are looking for,
such as variable bandwidth, RF section, 10 KHz whistle filter &
low impedance line driver output.
GOTO alt.binaries.schematics.electronic for the specs
& a schematic.
Yes. I have two of them. Not for sale.
Good Luck, John Stewart
Uncle Peter
" > > >>In article <G3OdnRd7ovd...@comcast.com>, "John Walton"
> > >><jdwa...@comcast.net> wrote:
> > >>
tending to be
> > higher at the top end because of the higher reactance (and therefore
> > higher ratio of reactance to resistance). So there'a compromise between
> > good sound but lousy selectivity at the low end, and good selectivity
> > but muddy sound at the high end. The AM broadcast band is bad that way,
> > since it's about a 3:1 frequency range.
>
It is also largely a function of Arithmetic Selectivity.
Pete
Steven Dinius
" Uncle Peter" <radiocon...@cox.netSPAM> wrote in message
news:keJOa.2582$mz6...@news2.east.cox.net...
My Halli has a knob for "sensitivity" that I'm guessing is more of a gain
boost.
Steven
Patrick Turner
Fred Nachbaur wrote:
Stagger tuning is a poor way of doing things in two cascaded stages,
and very hard to get right, since the IFT isn't tuned to centre on say 455 kHz,
but a few kHz to one side or the other.
The best way to increase AF BW is to effectivle over couple the first IFT,
whose response is then twin peaked, either side of 455 kHz cntre,
which compensates for the single broad peak the second IFT has.
The slope steepness away from the passband remain steep,
so for locals, thete is good AF BW and selectivity.
You can't do this with TRF.
Patrick Turner.
Patrick Turner
John Byrns wrote:
Interesting.
I have two tuned LC input circuits in the front end of my superhet reciever.
they are loosely coupled to the antenna.
I found that the BW was smallest at the lowest RF and there was significant side
band cutting
and would have been reduction of audio bandwidth, had I not used two LC coils, tuned
slightly apart, to give a wide enough BW.
It is necessary to have at least 60 dB attenuation when 40 kHz off the wanted F,
and 80 to 100 dB is better, and a couple of tuned circuit don't give enough
attenuation away from the wanted F, even if the LC circuit has a high Q.
If a single 455 kHz IF LC circuit has a BW of 20 kHz,
then the Q is only about 22.7.
When 4 such circuits are arrayed, the resultant Q is a lot more, and BW is a lot
less.
Such LC circuits have only say 15 dB attenuation at say 40 kHz off the wanted F,
so you need a few LC circuits to get stations 40 kHz away from the one you want to
be inaudible when there is silence in the programme.
We have a sports station of 300 watts at 1008 kHz, and
there is an entertainment station of 10,000 watts KHz less than a Km away,
at 1,053 kHz, and I know a radio is OK if I can tune to 1017,
and not hear the 1053 in the bacground of 1008 which
will be noisy, and distorted.
Patrick Turner.
Yves Monmagnon
"Patrick Turner" <in...@turneraudio.com.au> wrote in message
news:3F0B0FBB...@turneraudio.com.au...
Was just to satisfy my curiosity since have never seen such designs with
tubes, done !
I expected the synchrodyne was in place of the IF, that means a superhet
with synchrodyne detector.
Have done that (with chps, not tubes !) in a VHF traffic receiver to detect
SSB, AM and FM.
In FM, audio in the PLL error voltage.
We have chance that regulation authority left room for good (for me) radios
in FM band along all those
commercials playing heavy compressed beats !
> so more tubes are needed for a complex muting circuit.
> In this country, there were never any built commercially, afaik.
> Ppl stuck to plain old 5 valve superhets.
> Then eventually, after an absurdly long wait due to government bungles
> and
> vested interests in radio, we got FM in 1972, and the tube era was over
> then.
> The RDH4 has some FM circuits, which very few ppl could understand, let
> alone build.
> And very few know how multplex decoders work.
As an Ham, I explain multiplex to myself as a supressed carrier dual side
band 38khz, with a 19Khz pilot to rebuid the carrier.
Or even here !
>
> The Racal communications radio was one of the best ever made,
> with a cast and machined aluminium chassis, to seperate all the stages,
> and this had 22 valves.
> Digital techniques dominate HF radio now, and receivers just digitise the
> antenna signal,
> whatever it is, and get a computer to count what is in there, and just
> extract certain numbers,
> and there is your siganl, at any requency selected, its more like a noise
> analyser.
>
> I know hams who built everything in the shed, all the test gear,
> and all the transmitters and modulators, etc.
> They are mostly retired, and getting old, and they craft will die with
> them
MMMH ! I don't feel such old !! and not ready to die soon !!
> because HF radio is becoming an old fashioned way to communicate
> long distance.
> And that communication is subject to ionsphere condtions in 11 year
> cycles,
> and lots and lots of bloody noise and interference.
> Then there are all the problems of restrictions on radio gear in built up
> areas.
>
> I doubt I will ever become a ham radio guy.
> I find too much activity at AF frequencies, which prevents me going
> higher.
Yes, but we need DG to talk about what we're doing.
The main specificity of amateur radio is to use the rig to speech about the
rig !
And I will certainly never had the opportunity to listen your amps except if
you tie one as a modulator for an AM transmitter !
With those 12x6550, you could easily modulate a full RF KW .
Then I'll need a good variable selectivity receiver ;)
>
> Patrick Turner.
>
P.S.
Oh ! what a beautifull red tie and assorted handkerchief !!
Sure, with radios it's not so easy to have pictures from antipods !
>
Cheers, Yves.
Patrick Turner
Yves Monmagnon wrote:
> "Patrick Turner" <in...@turneraudio.com.au> wrote in message
> news:3F0B0FBB...@turneraudio.com.au...
> >
> >
> > Yves Monmagnon wrote:
> >
snip for brevity....
>
>
> Yes, but we need DG to talk about what we're doing.
> The main specificity of amateur radio is to use the rig to speech about the
> rig !
> And I will certainly never had the opportunity to listen your amps except if
> you tie one as a modulator for an AM transmitter !
> With those 12x6550, you could easily modulate a full RF KW .
> Then I'll need a good variable selectivity receiver ;)
Well maybe you will have to get busy to build the Miller TRF
reciever detailed at
http://users.rcn.com/jbyrns/Miller570.html
To make such a wonderful radio, REAL craftmanship is required.
>
> >
> > Patrick Turner.
> >
> P.S.
> Oh ! what a beautifull red tie and assorted handkerchief !!
> Sure, with radios it's not so easy to have pictures from antipods !
> >
> Cheers, Yves.
It was the only time I wore that suit last year......
Anyway, if I used the 300 watt amps to modulate a nice linear RF amp,
the atmospheric conditions between here and France would
ruin the fidelity.
But if I can borrow a few tubes from Radio America,
and do a deal with the power company, and borrow one of Radio National's
antennas, maybe you get me pretty good.
BTW, watch out for our Oz stars in Le Tour, I hear they are giving some guys
a hard time.
But maybe Lance will win again, who knows?
But that Miller radio is not a bad idea, and I tried a similar front end,
but its a bit hard to get the flat topped response.
The synchrodyne simply is supposed to add a constant level RF
tone of the same F as the recieved station, to that station signal.
Then all that is passed through a low pass filter, and hence audio is made.
if there was a station 9 kHz away, then you get a 9 kHz AF tone detected
in the audio from the wanted station.
Then if you have a very steep LP AF filter with a cut off of 8.5 kHz,
the adjoining carrier is kept out of the wanted station signal
But if the station at 9 kHz away from the wanted one has AF modulation,
then it is in the form of a 9 kHz carrier modulated by its AF content,
and the sharp cut off AF filter cannot keep out the sidebands either sider of 9
kHz,
and it sounds bloody awful.
So synchronous, or direct conversion is good where there are no
other strong signals, but needs to have a small AF BW where stations
of similar strength are close to each other.
I think a superhet is MUCH easier to build than that Miller radio.
And I still think 2.5 MHz IF would be practical, and have 3 IFTs.
This would allow a high Q for the total IF coils system,
and for 25 kHz AF BW, the Q could be 100.
And with a total of six tuned IF circuits, the slope away from the
cut off F would indeed be nice and steep.
There would not need to be much gain in the tubes used for IF.
The secert to getting clean performance from a frequency converter
is to have a fair degree of selectivity prior to the converter, so
the signal you want isn't modulated by strong stations nearby, also entering the
converter.
so the double LC tuned stage with mutual L or mutual C coupling
in the bottom of the L of each LC circuit
will make a nice front end system.
Patrick Turner.
JC
AM tuner from the 50's that was called hifi back then. I have never used it
so I can't comment on its sensitivity or selectivity. I have its companion
FM tuner the FM-3 and I used it use it several years ago it was a good
performer if you didn't need stereo. They are on ebay constantly here is
one
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3034424979&category=296
John Stewart
Heathkit also marketed another AM RX kit, their BC-1.
Has most of the features of the Eico HFT-94 with the exception
of the switchable IF BW. Not as well built either.
I have one here built by someone else. Bought it from one of
the guys who worked in the same lab as I about 40 years ago.
I used it here in the barn for several years.
Worth a look, anyway.
Cheers, John Stewart
John Byrns
<jc9...@navix.net> wrote:
Yeah, the heathkits are better receivers than the EICO HFT-94, although
they both suffer somewhat from the edgy sound of semiconductor diode
detectors. I had an HFT-94 in the early 1960's, and it just didn't do as
good a job as the heathkit does. There are at least 4 different models,
using similar circuitry, in the heathkit lineup of tube based AM tuners.
The famed BC-1A was the progenitor of the family, which also included the
AJ-20, AJ-21, and AJ-53. The BC-1A was built in the gold anodized
aluminum cabinet, which wasn't the greatest. The BC-1A had only a wide
band IF for High Fidelity, and didn't include the switch that was added in
the later models for switching to narrow band operation. The antenna
circuit also was changed in the later models, along with a change to solid
state rectification in the power supply, but otherwise the original BC-1A
circuit was carried through all four models. The AJ-20, which is the
model I have was built in the black and gold flecked cabinet, and has
improved chassis construction. The AJ-20 was re styled for the AJ-21 and
AJ-53 which used heathkit's "automotive dashboard" styling motif. The
AJ-21 and AJ-53 differed in the color scheme used.
It would be hard to beat one of these heatkit's, unless you can find a
Fisher AM-80 AM tuner. Although the original BC-1A is best avoided if you
are interested in DX reception.
Regards,
John Byrns
Robert Casey
>
>Yeah, the heathkits are better receivers than the EICO HFT-94, although
>they both suffer somewhat from the edgy sound of semiconductor diode
>detectors.
>
I have a BC-1A myself, and it uses semiconductor detector diodes. I
suppose I could
remove them and sub a tube like a 6110 or similar sub mini tube with
wire leads under
the chassis. It's sitting in storage right now, so it will be some time
before I can
do that....
Uncle Peter
"Robert Casey" <wa2...@ix.netcom.com> wrote in message
news:3F0DC684...@ix.netcom.com...
follower to drive a diode detector?
Pete
firedome
ever made in the early-mid 50s using a unique circuit that may be
found in the RDH3, I think...they put a huge engineering effort into
what was unfortunatley a dying art, they'd been building TRF and SH
radios for the military & commercial markets since the 20s...Audio
said it was the only AM made that approached FM quality - see the
upcoming article in Vacuum Tube Valley #20, coming out soon...
Roger in NY
" Uncle Peter" <radiocon...@cox.netSPAM> wrote in message news:<UwmPa.53$zd4.5@lakeread02>...
John Byrns
fire...@sover.net (firedome) wrote:
> Sargent-Rayment built what was considered the best commercial AM tuner
> ever made in the early-mid 50s using a unique circuit that may be
> found in the RDH3, I think...they put a huge engineering effort into
> what was unfortunatley a dying art, they'd been building TRF and SH
> radios for the military & commercial markets since the 20s...Audio
> said it was the only AM made that approached FM quality - see the
> upcoming article in Vacuum Tube Valley #20, coming out soon...
Who considered the S-R AM tuner "the best commercial AM tuner ever made in
the early-mid 50s"? There are many tuners that were just as good, and
some probably even better. What was unique about the S-R AM Tuner? With
the exception of the detector circuit it looks pretty ordinary. The
detector itself was not invented by S-R, but was invented by Selstead
(SP?) & Smith as discussed here in previous threads. The reference to
this detector can be found in the appendix to the RDH4, the RDH3 predates
the circuit.
Patrick Turner
John Byrns wrote:
The S&S circuit in the RDH4 appendix can easily be bettered.
You use a 12AU7 cathode follower, with its grid connected to the last IF coil,
and load the cathode with say 33k to ground, and have idle current
at 3 mA.
Then have a germaniun diode
with about half a mA of current flowing in it to ground, feeding a CR network.
The diode current keeps the diode turned on, so at 100% modulation,
or at very low signals, the diode distortion is extremely low,
and since the cathode follower sees a high value load, its voltage ability is
high,
so its thd is also low.
You should find that such a circuit should be able to produce an undistorted
15 vrms of signal if the AF detected is 1 kHz.
The maximum undistorted AF output voltage able to be detected falls
as the AF frequency rises.
But it sure beats all other simple detectors I have ever used.
I have not tried the use of a balanced IF amp, to eliminate
the 2H distortion so prevalent in a typical SE pentode stage
which is a variable mu type.
Then one could have a balanced CF detector arrangement,
and this would reduce thd to even lower.
If the IF envelope is amplified by an amp with a distorted transfer
characteristic, then the audio detected from it will contain the
the distortions.
Another method is to detect the +ve and -ve halves of the
AM envelope, and sum them to cancel thd.
The less thd you engineer into an AM tube radio,
the better they sound.
Patrick Turner.
John Byrns
<in...@turneraudio.com.au> wrote:
>
> I have not tried the use of a balanced IF amp, to eliminate
> the 2H distortion so prevalent in a typical SE pentode stage
> which is a variable mu type.
> Then one could have a balanced CF detector arrangement,
> and this would reduce thd to even lower.
> If the IF envelope is amplified by an amp with a distorted transfer
> characteristic, then the audio detected from it will contain the
> the distortions.
>
> Another method is to detect the +ve and -ve halves of the
> AM envelope, and sum them to cancel thd.
I'm not sure if I am following this last correctly, I may have to think
about it some more to be sure, but it sounds like you suggesting this as a
way to cancel the some of the distortion when a variable mu pentode stage
is used to drive the diode? Of course some AM tuners eliminate this
problem at the source by using an ordinary pentode, no variable mu, to
drive the diode. Would the full wave voltage doubler detector used in the
Heathkit tuners accomplish this same goal, it's a possible advantage for
this circuit I hadn't thought of before? No wonder the Heathkits are so
great!
4seasonphoto
required tuning capacitor and transformers, except by scrounging at
hamfests and the like.
I've been very happy with my McIntosh FM tuner, and had figured on
eventually picking up a mono AM/FM Mac. Nothing against having one which
includes the multiplex adapter, but I figure folks will be more willing
to make me a deal if it's missing ;-) As the various disc and mica caps,
carbon comp resistors, original wires and connectors seemed to be in
good shape, I left them alone. Electrolytics and paper caps were
suspect, and the selenium rectifier was weak, so I replaced these with
modern audiophile-type parts, and all's been fine. Some folks carefully
pull the guts out of vintage caps and restuff them with modern parts in
order to retain the vintage look inside and out. But there was no way I
could do that when replacing 10 uf electrolytic coupling caps with 10 uf
Solen film caps!
But if I were really *really* serious about wanting the best possible AM
for all situations, including DXing, maybe I'd look into a "boat anchor"
general coverage receiver from the past, and combine that with a modern
synchronous detector with selectable sidebands. And I'd ensure that it
offered at least one wide-bandwidth setting.
Patrick Turner
John Byrns wrote:
> In article <3F0EE9B7...@turneraudio.com.au>, Patrick Turner
> <in...@turneraudio.com.au> wrote:
>
> >
> > I have not tried the use of a balanced IF amp, to eliminate
> > the 2H distortion so prevalent in a typical SE pentode stage
> > which is a variable mu type.
> > Then one could have a balanced CF detector arrangement,
> > and this would reduce thd to even lower.
> > If the IF envelope is amplified by an amp with a distorted transfer
> > characteristic, then the audio detected from it will contain the
> > the distortions.
> >
> > Another method is to detect the +ve and -ve halves of the
> > AM envelope, and sum them to cancel thd.
>
> I'm not sure if I am following this last correctly, I may have to think
> about it some more to be sure, but it sounds like you suggesting this as a
> way to cancel the some of the distortion when a variable mu pentode stage
> is used to drive the diode? Of course some AM tuners eliminate this
> problem at the source by using an ordinary pentode, no variable mu, to
> drive the diode.
In MY home designed and built from scratch superhet radio,
I have a 6BX6 as the IF tube, with an unbypassed Rk
to provide some current FB.
The linearity at 10vrms of IF signal is fairly good for a
sharp cut off pentode in this application, and its better than say a 6BA6.
The detector I have produces its AF from the +ve
side of the IF envelope, but also another cap and diode is used to
produce an AVC -voltage from the -ve side of the envelope,
and this AVC is only applied to an RF input stage
using a variable mu twin triode in a grounded grid stage, driven by a CF.
The signal is so small in the triode stage that its linearity is fine,
and so is that of the 6AN7 converter stage, which like the 6BX6,
runs with fixed bias.
> Would the full wave voltage doubler detector used in the
> Heathkit tuners accomplish this same goal, it's a possible advantage for
> this circuit I hadn't thought of before? No wonder the Heathkits are so
> great!
I don't know.
I haven't seen the schematic and only careful analysis,
or trial and measurement would tell you.
But if you have the last IFT winding set up with 1 megohm
from each end to some fixed positive bias of say +100v,
then the grids of two CFs can be direct connected to each side of the winding.
Then there are two seperate diode detectors, and filters, and another pair of
CF,
with an AF 1:1 tranny attatched, and from its secondary there will be little
thd
when the balanced output is coverted to single phase for use with a following
amplifier,
if of course it needs an unbalanced input signal.
Balanced all the way, including a balanced RF input stage and converter stage
has some merits.
You just use twice the tube count, but plenty of tubes are there to use.
The 1M resistors won't load down the last IF tube significantly,
or over damp the Q of the last tuned circuit.
In FM IF circuits, no need for balanced amps,
because the linearity don't matter much, since the limiter output
is just a 10.7 MHz signal which is devoid of amplitude changes,
and distortion products of 10.7 MHz envelope can't
appear in the audio, since the detector only
detects variations in the distances between the
up/downs of the 10.7 MHz waves.
Patrick Turner.
John Byrns
<in...@turneraudio.com.au> wrote:
> John Byrns wrote:
>
> > In article <3F0EE9B7...@turneraudio.com.au>, Patrick Turner
> > <in...@turneraudio.com.au> wrote:
> >
> > >
> > > I have not tried the use of a balanced IF amp, to eliminate
> > > the 2H distortion so prevalent in a typical SE pentode stage
> > > which is a variable mu type.
> > > Then one could have a balanced CF detector arrangement,
> > > and this would reduce thd to even lower.
> > > If the IF envelope is amplified by an amp with a distorted transfer
> > > characteristic, then the audio detected from it will contain the
> > > the distortions.
> > >
> > > Another method is to detect the +ve and -ve halves of the
> > > AM envelope, and sum them to cancel thd.
> >
> > I'm not sure if I am following this last correctly, I may have to think
> > about it some more to be sure, but it sounds like you suggesting this as a
> > way to cancel the some of the distortion when a variable mu pentode stage
> > is used to drive the diode? Of course some AM tuners eliminate this
> > problem at the source by using an ordinary pentode, no variable mu, to
> > drive the diode.
I have had a chance to think about this a little more, and pending doing
the hard math, I don't believe that using a push pull IF stage will help
with "the 2H distortion so prevalent in a typical SE pentode stage which
is a variable mu type." Push pull stages only cancel even order
distortion products, and the following IF transformer also removes the
even order distortion products, including "2H", so a push pull IF stage
will only help in that it will provide more power to drive the detector
diode, which with your cathode follower scheme is pretty much irrelevant
anyway.
> In MY home designed and built from scratch superhet radio,
> I have a 6BX6 as the IF tube, with an unbypassed Rk
> to provide some current FB.
> The linearity at 10vrms of IF signal is fairly good for a
> sharp cut off pentode in this application, and its better than say a 6BA6.
> The detector I have produces its AF from the +ve
> side of the IF envelope, but also another cap and diode is used to
> produce an AVC -voltage from the -ve side of the envelope,
> and this AVC is only applied to an RF input stage
> using a variable mu twin triode in a grounded grid stage, driven by a CF.
> The signal is so small in the triode stage that its linearity is fine,
> and so is that of the 6AN7 converter stage, which like the 6BX6,
> runs with fixed bias.
>
> > Would the full wave voltage doubler detector used in the
> > Heathkit tuners accomplish this same goal, it's a possible advantage for
> > this circuit I hadn't thought of before? No wonder the Heathkits are so
> > great!
>
> I don't know.
> I haven't seen the schematic and only careful analysis,
> or trial and measurement would tell you.
As above, I have decided it wouldn't help because the IFT has already
filtered out the 2H distortion.
> But if you have the last IFT winding set up with 1 megohm
> from each end to some fixed positive bias of say +100v,
> then the grids of two CFs can be direct connected to each side of the winding.
>
> Then there are two seperate diode detectors, and filters, and another pair of
> CF,
> with an AF 1:1 tranny attatched, and from its secondary there will be little
> thd
> when the balanced output is coverted to single phase for use with a following
> amplifier,
> if of course it needs an unbalanced input signal.
This might help with distortion generated in your first CF, but why not
just build a more robust CF in the first place? On the other hand, it
isn't clear to me that you really want to drive the diode with a CF in the
first place, what is wrong with hooking it directly to the IFT in the
traditional fashion? A CF following the diode is definitely a good thing
though.
> Balanced all the way, including a balanced RF input stage and converter stage
> has some merits.
> You just use twice the tube count, but plenty of tubes are there to use.
> The 1M resistors won't load down the last IF tube significantly,
> or over damp the Q of the last tuned circuit.
I don't see the advantage of "balanced all the way" as far as audio
distortion goes?
> In FM IF circuits, no need for balanced amps,
> because the linearity don't matter much, since the limiter output
> is just a 10.7 MHz signal which is devoid of amplitude changes,
> and distortion products of 10.7 MHz envelope can't
> appear in the audio, since the detector only
> detects variations in the distances between the
> up/downs of the 10.7 MHz waves.
I'm not an expert in this, but I think this is a fallacy, and the
linearity of FM IF stages is actually quite important. Nonlinear IF
stages in an FM system that sees only one signal may very well not effect
the detected audio, but I believe that nonlinear IF stages can degrade the
capture ratio, and affect the performance of the FM receiver when it
confronted with multiple signals, such as under multipath conditions, or
conditions of co channel interference, but there are many complexities
that enter into this.
firedome
> In article <c534ce0f.0307...@posting.google.com>,
> fire...@sover.net (firedome) wrote:
>
> > Sargent-Rayment built what was considered the best commercial AM tuner
> > ever made in the early-mid 50s using a unique circuit that may be
> > found in the RDH3, I think...they put a huge engineering effort into
> > what was unfortunatley a dying art, they'd been building TRF and SH
> > radios for the military & commercial markets since the 20s...Audio
> > said it was the only AM made that approached FM quality - see the
> > upcoming article in Vacuum Tube Valley #20, coming out soon...
>
> Who considered the S-R AM tuner "the best commercial AM tuner ever made in
> the early-mid 50s"? There are many tuners that were just as good, and
> some probably even better. What was unique about the S-R AM Tuner?
Who considered the S-R tuner the best commercial tuner ever made?...
Audio Engineering (July 1953)..."there has never before been available
a radio receiver which provided AM quality which approached FM" ...and
Consumer Research Bulletin (March 1953)..."the first satisfactory
detector circuit yet observed for high fidelity use in an AM tuner".
As for the circuit's provenance, I never said S-R invented it...it
was, as I said, an adaptation of the Selden -Smith 2 tube decector
circuit (did any one else use 2 tube detectors?) found in the RDH3.
Will Rayment never claimed to have invented it, but was the only one
to use it in a commercial product AFAIK. It also had IF coils built on
an S-R designed "honeycomb" winding machine so superior that McIntosh
tried (and failed) to duplicate it when bringing out their first tuner
(MR-55) and they subsequently approached Will Rayment for the rights
to use it. Check out a proprerly restored SR-51 for AM performance. I
believe you will be impressed.
Roger In NY
John Byrns
> Who considered the S-R tuner the best commercial tuner ever made?...
> Audio Engineering (July 1953)..."there has never before been available
> a radio receiver which provided AM quality which approached FM" ...and
> Consumer Research Bulletin (March 1953)..."the first satisfactory
> detector circuit yet observed for high fidelity use in an AM tuner".
Those are certainly some fantastic superlatives!
> As for the circuit's provenance, I never said S-R invented it...
And for my part I never said you said S-R invented it.
> it was, as I said, an adaptation of the Selden -Smith 2 tube decector
> circuit (did any one else use 2 tube detectors?) found in the RDH3.
I missed the part where you said that. The Ampex AM/FM simulcast stereo
tuner also used a similar "two tube" detector circuit. I forget the
actual Ampex model number of this tuner, but many people refer to it as
the "008" tuner, but that is not the actual model number.
What chapter and page of the RDH3 is it on? I thought it only made it
into an appendix of the RDH4, probably after the S-R tuner using it came
out.
> Will Rayment never claimed to have invented it, but was the only one
> to use it in a commercial product AFAIK.
Ampex also used it in their AM/FM stereo tuner back in the late 50s.
> It also had IF coils built on
> an S-R designed "honeycomb" winding machine so superior that McIntosh
> tried (and failed) to duplicate it when bringing out their first tuner
> (MR-55) and they subsequently approached Will Rayment for the rights
> to use it.
Exactly what is the advantage of this "honeycomb" construction method over
other IF transformer construction techniques, say for example the cup core
style of construction? Why was McIntosh so interested in the "honeycomb"
design? I would guess that the primary advantage of the "honeycomb"
construction in a simple double tuned IF transformer is not technical, but
is in the marketing area, giving the manufacturer another unusual point to
advertise.
> Check out a proprerly restored SR-51 for AM performance. I
> believe you will be impressed.
OK, but they are had to find, S-R didn't exactly take the world by storm
with their tuner, where can I find one today? Is the SR-51 the AM only
version? It might be marginally easier to find one of the AM/FM models,
do those also impress?
Patrick Turner
John Byrns wrote:
I assure you there are references to IMD caused by IF amp nonlinearities.
Maybe you should inspect the output of a typical pentode IF amp.
You may not easily see less than 2% thd on the modulation waveform on a CRO.
Perahps you ought to measure a typical IFamp, by using a linear detector, as
described.
But I am sure that a balanced amp with pentodes, with a common cathode.
each with a lot of 2H when used alone as an SE amp, would display
less thd than the SE case.
They use PP balanced oscillators when a signal output
is required to be cleaner than an SE type.
>
>
> > In MY home designed and built from scratch superhet radio,
> > I have a 6BX6 as the IF tube, with an unbypassed Rk
> > to provide some current FB.
> > The linearity at 10vrms of IF signal is fairly good for a
> > sharp cut off pentode in this application, and its better than say a 6BA6.
> > The detector I have produces its AF from the +ve
> > side of the IF envelope, but also another cap and diode is used to
> > produce an AVC -voltage from the -ve side of the envelope,
> > and this AVC is only applied to an RF input stage
> > using a variable mu twin triode in a grounded grid stage, driven by a CF.
> > The signal is so small in the triode stage that its linearity is fine,
> > and so is that of the 6AN7 converter stage, which like the 6BX6,
> > runs with fixed bias.
> >
> > > Would the full wave voltage doubler detector used in the
> > > Heathkit tuners accomplish this same goal, it's a possible advantage for
> > > this circuit I hadn't thought of before? No wonder the Heathkits are so
> > > great!
> >
> > I don't know.
> > I haven't seen the schematic and only careful analysis,
> > or trial and measurement would tell you.
>
> As above, I have decided it wouldn't help because the IFT has already
> filtered out the 2H distortion.
If you examine the plate output of a pentode IF amp,
with a AM envelope, the audio signal outlines the extent of RF voltage swing
on the top of the wave form, and on the bottom, but the two AF
modulation waves are out of phase.
The upper half of the envelope is amplified more than the bottom half, since
the Gm of the tube is higher when more instantaneous current flows.
The AF envelope is thus endowed with the the tube transfer characteristics
and the AF recovered by the following detector will contain such distortions,
and with an SE stage, there is a lot of 2H introduced to the recovered AF,
and as well some other H because a pentode generates a lot of thd
at high outputs.
The impedance of the IFT input might be 50k at Fo, but
at AF, its maybe only 100 ohms, so any even order assymetry in the top and bottom
halves
of the envelope wave form tends to balance out.
But the damage to the fidelity has already been done by generation of imds.
I can't see why ppl should try a balanced RF and IF and oscillator,
just to see how clean they could get a tube stage to be.
I don't believe they would be dissappointed.
Just as we tryto get thd of an audio amp down to a respectable
0.1%, then why not aim in the same direction for all our tuners,
where it is applicable?
>
>
> > But if you have the last IFT winding set up with 1 megohm
> > from each end to some fixed positive bias of say +100v,
> > then the grids of two CFs can be direct connected to each side of the winding.
> >
> > Then there are two seperate diode detectors, and filters, and another pair of
> > CF,
> > with an AF 1:1 tranny attatched, and from its secondary there will be little
> > thd
> > when the balanced output is coverted to single phase for use with a following
> > amplifier,
> > if of course it needs an unbalanced input signal.
>
> This might help with distortion generated in your first CF, but why not
> just build a more robust CF in the first place?
Mine is as robust as I think I need it to be.
All I am saying, it could all be better, in terms of
measured imd, and that can't be a bad thing
> On the other hand, it
> isn't clear to me that you really want to drive the diode with a CF in the
> first place, what is wrong with hooking it directly to the IFT in the
> traditional fashion?
The load seen by the IF amp is the impedance of the LC circuit of the IFT
primary, plus the reflected load of the secondary multiplied
by the mutual coupling factor, usually a lot less than 1.0.
Nevertheless, we realise the best IFT/tube performance
if the load driven by the IFT is as high as possible a value.
The CF input grid is high enough.
So the IFT and IF amp tube are buffered from the
clipping of the peaks of the envelope due to the diode charging.
The CF itself sees a benign load, and the diiode is biased on,
and I assure you its a more precise way to demodulate an AM envelope.
There is less latch up of the CR network being charged by the CF,
because the CF has plenty of current, and is a low Ro
sigbnal source.
> A CF following the diode is definitely a good thing
> though.
>
> > Balanced all the way, including a balanced RF input stage and converter stage
> > has some merits.
> > You just use twice the tube count, but plenty of tubes are there to use.
> > The 1M resistors won't load down the last IF tube significantly,
> > or over damp the Q of the last tuned circuit.
>
> I don't see the advantage of "balanced all the way" as far as audio
> distortion goes?
Maybe you should try building one to find out.
But I have schematics using twin gated fets for RF input,
and for a balanced converter, and there is no reason why it couldn't be
balanced all the way through, and benefit from the greater dynamic range of tubes.
>
>
> > In FM IF circuits, no need for balanced amps,
> > because the linearity don't matter much, since the limiter output
> > is just a 10.7 MHz signal which is devoid of amplitude changes,
> > and distortion products of 10.7 MHz envelope can't
> > appear in the audio, since the detector only
> > detects variations in the distances between the
> > up/downs of the 10.7 MHz waves.
>
> I'm not an expert in this, but I think this is a fallacy, and the
> linearity of FM IF stages is actually quite important.
Nope, and a good FM IF stage starts to limit whenh a low threshold of input signal
is applied,
and the output of the limiter, often a 6AU6 is that of a tube enduring
30 dB of gross overload, ie, the tube is being driven well
into clipping, and it it wasn't for the fact of the discriminator LC input circuit,
the wave form at the 6AU6 would be a 10.7 MHz square wave.
The discriminator would still only produce the same audio,
even with a perfectly square IF wave, since the frequency changes of +/- 75 kHz
carry the audio info and that's all that the discriminator detects,
unless some unwanted AM is present, such as noise.
FM is inherently immune to noise, such as lightning bolts,
which rarely manifests itself
as FM, but usually only as a burst of AM.
> Nonlinear IF
> stages in an FM system that sees only one signal may very well not effect
> the detected audio, but I believe that nonlinear IF stages can degrade the
> capture ratio, and affect the performance of the FM receiver when it
> confronted with multiple signals, such as under multipath conditions, or
> conditions of co channel interference, but there are many complexities
> that enter into this.
Perhaps so, but I doubt that matters if the IF is receieving
enough signal to drive it well into limiting.
If the limiting was marginal, and some other received out of phase signal
could beat with the wanted signal, causing the IF to drop below limiting, and then
subject
the discriminator to AM, and intermittent AM, and the sound
goes scratchy, and distorted.
Patrick Turner.
Randy and/or Sherry
John Byrns wrote:
> Exactly what is the advantage of this "honeycomb" construction method over
> other IF transformer construction techniques, say for example the cup core
> style of construction? Why was McIntosh so interested in the "honeycomb"
> design? I would guess that the primary advantage of the "honeycomb"
> construction in a simple double tuned IF transformer is not technical, but
> is in the marketing area, giving the manufacturer another unusual point to
> advertise.
As you suspect - the primary "advantage" was in print. Having said that
- I'll noted that there was a school of thought that since the wires in
a honeycomb coil weren't "wound tightly together side-by side and in
tight layers"- the capacitance between layers / adjacent windings was
less - therefore providing a more "pure" inductance.
best regards...
--
randy guttery
A Tender Tale - a page dedicated to those Ships and Crews
so vital to the United States Silent Service:
http://tendertale.com
John Byrns
> As for the circuit's provenance, I never said S-R invented it...it
> was, as I said, an adaptation of the Selden -Smith 2 tube decector
> circuit (did any one else use 2 tube detectors?) found in the RDH3.
> Will Rayment never claimed to have invented it, but was the only one
> to use it in a commercial product AFAIK.
I tried to email you about some information that I received from Walter
Selsted on the "two tube" detector, but your email address bounces.
Perhaps you already talked to Walter, did you interview him for your
article on the S-R tuner?
John Byrns
<in...@turneraudio.com.au> wrote:
> John Byrns wrote:
>
> > In article <3F0EF457...@turneraudio.com.au>, Patrick Turner
> > <in...@turneraudio.com.au> wrote:
> > I have had a chance to think about this a little more, and pending doing
> > the hard math, I don't believe that using a push pull IF stage will help
> > with "the 2H distortion so prevalent in a typical SE pentode stage which
> > is a variable mu type." Push pull stages only cancel even order
> > distortion products, and the following IF transformer also removes the
> > even order distortion products, including "2H", so a push pull IF stage
> > will only help in that it will provide more power to drive the detector
> > diode, which with your cathode follower scheme is pretty much irrelevant
> > anyway.
>
> I assure you there are references to IMD caused by IF amp nonlinearities.
> Maybe you should inspect the output of a typical pentode IF amp.
> You may not easily see less than 2% thd on the modulation waveform on a CRO.
> Perahps you ought to measure a typical IFamp, by using a linear detector, as
> described.
Read what I said again, I didn't say that there is no "IMD" caused by the
IF amp nonlinearities, what I said was that the even order distortion
products, including "2H", are filtered out by the IF transformer before
reaching the detector.
What mechanism are you suggesting that allows a significant amount of the
even order distortion products make it through the IF transformer?
As far as measuring the distortion at the detector output, I don't have an
AM generator with sufficiently low distortion to do this, I wish that I
did have a generator good enough to measure the effects we are talking
about.
> But I am sure that a balanced amp with pentodes, with a common cathode.
> each with a lot of 2H when used alone as an SE amp, would display
> less thd than the SE case.
This is not a "SE" audio amplifier, this is an IF amplifier, which in a
tube radio is typically followed by a double tuned IF transformer. The
double tuned IF transformer filters out the even order distortion
products, allowing only the odd order distortion to pass, just like your
push pull stage. The only thing the push pull IF stage is going to do is
increase the power available to drive the detector.
> They use PP balanced oscillators when a signal output
> is required to be cleaner than an SE type.
An oscillator is a whole different matter, we don't want oscillator
harmonics, or RF harmonics, getting into the mixer where they can create
spurious responses.
> > > In FM IF circuits, no need for balanced amps,
> > > because the linearity don't matter much, since the limiter output
> > > is just a 10.7 MHz signal which is devoid of amplitude changes,
> > > and distortion products of 10.7 MHz envelope can't
> > > appear in the audio, since the detector only
> > > detects variations in the distances between the
> > > up/downs of the 10.7 MHz waves.
> >
> > I'm not an expert in this, but I think this is a fallacy, and the
> > linearity of FM IF stages is actually quite important.
>
> Nope,
I suggest you read up on your FM theory, there were a lot of good papers
published on this subject in the 1950s.
Uncle Peter
"Randy and/or Sherry" <comc...@mississippi.net> wrote in message
news:3F104B64...@mississippi.net...
>
> > As you suspect - the primary "advantage" was in print. Having said that
> - I'll noted that there was a school of thought that since the wires in
> a honeycomb coil weren't "wound tightly together side-by side and in
> tight layers"- the capacitance between layers / adjacent windings was
> less - therefore providing a more "pure" inductance.
>
> best regards...
> --
> randy guttery
>
> A Tender Tale - a page dedicated to those Ships and Crews
> so vital to the United States Silent Service:
> http://tendertale.com
Randy
I remember reading somewhere that one of the advantages to that
sort of winding was to reduce the proximity effect of having wires
laying to close to each other, which would cause a loss of Q?
Pete
>
donut
Parts are still readily available. I remember the original poster being
concerned about the tuning cap. Antique Electronic Supply has a beautiful 3
gang 500 pf/gang unit for $24.95.
Coils and IF transformers are all easily available, and for not much cash,
either. The Miller coils are available as NOS from a number of places. AES
has coils and IF transformers.
Tubes are easy. Using the BA6/BE6 lineup would be a snap. The tubes are
about $3 or $4 each.
What I'm saying is that the electronic part of the project is easy, and not
all that much more expensive than 25 years ago.
Everything is PC mount now. No socket hole punches needed.
It seems like actually deciding how to build the thing is the biggest
problem! ;)
Hardware (chassis, dial, etc.) will be much harder than building the
electronics.
I'm a big fan of the modular technique. You work with smaller boards, and
build in steps. Each module can be tested separately.
It was always my dream from a very young age to build my own receiver,
which is why I've been following this.
John Byrns
> Coils and IF transformers are all easily available, and for not much cash,
> either. The Miller coils are available as NOS from a number of places. AES
> has coils and IF transformers.
Where are some places I can get NOS Miller coils and transformers?
Randy and/or Sherry
donut wrote:
> What I'm saying is that the electronic part of the project is easy, and not
> all that much more expensive than 25 years ago.
>
> Everything is PC mount now. No socket hole punches needed.
As I noted before - my senior class project was building an AM radio.
And the "idea" was to build it in sections... power supply / audio then
IF and converter. I just built mine in a straight line layout - used the
brake in the sheet metal shop to form the chassis - chassis punches for
the tube sockets and a bit of careful drilling / filing for the IF can
mounts. It was a "simple" AA5 - but it worked fine (for an AA5+1 - I
added a 6E5 to the lineup - just "because"). It wasn't fancy - but
looked as good as any "factory" chassis - I guess I could have chrome
plated it --- since I was getting some stuff for my car chromed about
the same time - probably wouldn't have been that big a deal just never
crossed my mind. Point is - with a little effort - standard chassis
work is certainly within the reach of anyone willing to do just a bit of
work - either starting with a "bud box" or forming one from scratch.
Unless someone decides to get a bit funky - you only need one size of
chassis punch (either for miniature or octal tubes)... usually for less
than $30.00 - or "borrowed" from a friendly electrician.
just my .02...
Patrick Turner
John Byrns wrote:
> In article <3F102EEA...@turneraudio.com.au>, Patrick Turner
> <in...@turneraudio.com.au> wrote:
>
> > John Byrns wrote:
> >
> > > In article <3F0EF457...@turneraudio.com.au>, Patrick Turner
> > > <in...@turneraudio.com.au> wrote:
>
> > > I have had a chance to think about this a little more, and pending doing
> > > the hard math, I don't believe that using a push pull IF stage will help
> > > with "the 2H distortion so prevalent in a typical SE pentode stage which
> > > is a variable mu type." Push pull stages only cancel even order
> > > distortion products, and the following IF transformer also removes the
> > > even order distortion products, including "2H", so a push pull IF stage
> > > will only help in that it will provide more power to drive the detector
> > > diode, which with your cathode follower scheme is pretty much irrelevant
> > > anyway.
> >
> > I assure you there are references to IMD caused by IF amp nonlinearities.
> > Maybe you should inspect the output of a typical pentode IF amp.
> > You may not easily see less than 2% thd on the modulation waveform on a CRO.
> > Perahps you ought to measure a typical IFamp, by using a linear detector, as
> > described.
>
> Read what I said again, I didn't say that there is no "IMD" caused by the
> IF amp nonlinearities, what I said was that the even order distortion
> products, including "2H", are filtered out by the IF transformer before
> reaching the detector.
Indeed, if the top half of the IF envelope is amplified more than the bottom
half, then the waveform is as if somebody added an AF component to
the 455 kHz wave form, and this is shunted by the low impedance of the
IFTs at AF.
But the IMDs remain, and if the envelope is subject to limiting
or clipping then this will distort the detected AF.
If the peaks of an AM envelope are compressed on top and bottom,
then that's odd order distortion of the whole wave form,
but when one side of the compressed IF envelope is detected, the
AF thd is mainly even order!
In most sets the output voltage from the detector is around
an average of 1 to 2 vrms, from a 30% modulated carrier.
Working back from that, the peak to peak envelope
of the IF amp has to be about 8 to 16 v.
The maximum before IF amp clipping occurs could be
perhaps 100v, at which voltage the thd is severe.
Luckily, tube thd at lower voltage outputs is not severe,
but nevertheless, the more linear the IF stage, the
less thd there is in the AF.
>
>
> What mechanism are you suggesting that allows a significant amount of the
> even order distortion products make it through the IF transformer?
I agree, the even order harmonic mauling of the modulation don't make it through.
But the IMDs must surely get through, as would any odd order
harmonic mauling of the wave form.
>
>
> As far as measuring the distortion at the detector output, I don't have an
> AM generator with sufficiently low distortion to do this, I wish that I
> did have a generator good enough to measure the effects we are talking
> about.
I do have enough gear make some measurements.
My RF gene has an SE pentode with modulated AM output,
with an RF output transformer, and there is some
even order generated on the shape of the modulation envelope.
But with a dual trace CRO, I can monitor the input AM wave form,
and overlay the detected AF on top and adjust until they line up very closely
at some low value of AM input to the set under test, and with the AVC shunted to
a fixed negative voltage which is usual for most local recieved stations.
Then when the input AM is increased, there should be an eqaul increase
in the detected AF, free of much difference between the AF and the
modulation envelope shape.
If about 15vrms at 1 kHz can be detected before
any serious distortions occur, you have a good RF input, mixer, IF amp, and
detector.
I don't have a really pure sig gene, with a certified less than 0.01% thd.
So just measuring the thd in the AF output from a detector
doesn't tell you much.
But if you do have less than 1% at say 10v output, which ius impossible to see
on a CRO, then both the sig gene, and radio are doing OK.
I am simply trying to suggest that there may be some benefits, even though small,
to using balanced circuitry right through an AM radio reciever,
even up to the audio amp speaker.
>
>
> > But I am sure that a balanced amp with pentodes, with a common cathode.
> > each with a lot of 2H when used alone as an SE amp, would display
> > less thd than the SE case.
>
> This is not a "SE" audio amplifier,
I didn't say it was.
> this is an IF amplifier, which in a
> tube radio is typically followed by a double tuned IF transformer. The
> double tuned IF transformer filters out the even order distortion
> products, allowing only the odd order distortion to pass, just like your
> push pull stage. The only thing the push pull IF stage is going to do is
> increase the power available to drive the detector.
But what of the imd generated by the non linear operation of the SE tube?
RDH4 does mention the problems I have mentioned on thr bottom
of page 1,067.
Using a PP balanced amp for at least the final IF amp
would have nothing but a beneficial effect on the sound,
and I suggest ppl try it.
The extra 6 dB of voltage ability would be welcome.
But of course unless the stage is done with an IFT with centre tapped
windings, then parafeed might have to be used, with a centre tapped RF choke used
to
feed the DC to each half of the PP IF amp.
The CF buffers following the IF amp can be quite linear,
and if 12AT7 is used, the thd at 10v output would be indeed low,
even if there is a simple SE detector. The gain reduction
in a a 12AT7 CF is about 50 times, so the thd introduced is
0.2% maximum. The diode distortion following is lowest when Vo is high.
>
>
> > They use PP balanced oscillators when a signal output
> > is required to be cleaner than an SE type.
>
> An oscillator is a whole different matter, we don't want oscillator
> harmonics, or RF harmonics, getting into the mixer where they can create
> spurious responses.
Agreed.
>
>
> > > > In FM IF circuits, no need for balanced amps,
> > > > because the linearity don't matter much, since the limiter output
> > > > is just a 10.7 MHz signal which is devoid of amplitude changes,
> > > > and distortion products of 10.7 MHz envelope can't
> > > > appear in the audio, since the detector only
> > > > detects variations in the distances between the
> > > > up/downs of the 10.7 MHz waves.
> > >
> > > I'm not an expert in this, but I think this is a fallacy, and the
> > > linearity of FM IF stages is actually quite important.
> >
> > Nope,
>
> I suggest you read up on your FM theory, there were a lot of good papers
> published on this subject in the 1950s.
But what about when the amp is under serious limiting?
The IF amp is then saturated, and the current distortion is huge.
The whole stage simply acts to convert the incomming IF wave a
simple clipped sine wave, and the IF can be quite non linear in this condition,
when AF fidelity is then highest.
FM radios represented nice big move away from the constrictions we
need to carefully follow in AM recievers, where any FM would be deadly.
Patrick Turner.
donut
74-132.d.enteract.com:
> Where are some places I can get NOS Miller coils and transformers?
AES has some. Also I found:
firedome
> In article <c534ce0f.0307...@posting.google.com>,
> fire...@sover.net (firedome) wrote:
>
> > Who considered the S-R tuner the best commercial tuner ever made?...
> > Audio Engineering (July 1953)..."there has never before been available
> > a radio receiver which provided AM quality which approached FM" ...and
> > Consumer Research Bulletin (March 1953)..."the first satisfactory
> > detector circuit yet observed for high fidelity use in an AM tuner".
>
> Those are certainly some fantastic superlatives!
>
> > As for the circuit's provenance, I never said S-R invented it...
>
> And for my part I never said you said S-R invented it.
>
> > it was, as I said, an adaptation of the Selden -Smith 2 tube decector
> > circuit (did any one else use 2 tube detectors?) found in the RDH3.
>
> I missed the part where you said that. The Ampex AM/FM simulcast stereo
> tuner also used a similar "two tube" detector circuit. I forget the
> actual Ampex model number of this tuner, but many people refer to it as
> the "008" tuner, but that is not the actual model number.
I mentioned the circuit in RDH3, not the inventors names... I'm going
on what Will Rayment told me when I interviewed him for VTV, and these
are his recollections of what happened 50 yrs ago. McIntosh obviously
thought highly of the S-R IF coils as they pursued it with diligence.
I myself am not sure of the advantage of honeycomb construction, but
McIntosh apparently thought there was one beyond marketing hype. I'll
ask Sid Corderman if he knows when I interview him...As for SR-51s,
there was one on eBay a few months ago, and I believe the SR-68 and
later SR-1000 had essentially the same circuit. There was an SR-1000
on eBay a couple weeks ago in CA that didn't sell, maybe you can
contact them and make an offer. I'm told by Earl Yarrow it is an
excellent tuner.
>
> What chapter and page of the RDH3 is it on? I thought it only made it
> into an appendix of the RDH4, probably after the S-R tuner using it came
> out.
>
> > Will Rayment never claimed to have invented it, but was the only one
> > to use it in a commercial product AFAIK.
>
> Ampex also used it in their AM/FM stereo tuner back in the late 50s.
I wasn't aware of the Ampex.
> > It also had IF coils built on
> > an S-R designed "honeycomb" winding machine so superior that McIntosh
> > tried (and failed) to duplicate it when bringing out their first tuner
> > (MR-55) and they subsequently approached Will Rayment for the rights
> > to use it.
>
> Exactly what is the advantage of this "honeycomb" construction method over
> other IF transformer construction techniques, say for example the cup core
> style of construction? Why was McIntosh so interested in the "honeycomb"
> design? I would guess that the primary advantage of the "honeycomb"
> construction in a simple double tuned IF transformer is not technical, but
> is in the marketing area, giving the manufacturer another unusual point to
> advertise.
>
> > Check out a proprerly restored SR-51 for AM performance. I
> > believe you will be impressed.
>
> OK, but they are had to find, S-R didn't exactly take the world by storm
> with their tuner, where can I find one today? Is the SR-51 the AM only
> version? It might be marginally easier to find one of the AM/FM models,
> do those also impress?
>
>
> Regards,
>
> John Byrns
Roger Anderson
Lake Champlain NY
Uncle Peter
"John Byrns" <jby...@rcn.com> wrote in message
news:jbyrns-1207...@216-80-74-132.d.enteract.com...
>
>
> Regards,
>
> John Byrns
>
>
A dwindling supply was offered here... Not much left..
http://www.metcoelectronics.com/coils.htm#antenna
Peter
John Byrns
Thanks, I will check the "oldradioparts" site, I never noticed that AES
had any of the J.W. Miller IF Transformers that I am interested in.
John Byrns
<in...@turneraudio.com.au> wrote:
"2H" distortion is just a special case of 2nd order "IMD". In looking up
your reference to the RDH4th, I found that the page you referenced doesn't
really say much, but it does refer in turn to pages 944 and 945, which
briefly mentions that the transfer characteristic of the vari-mu tube can
be modeled by a power series, and that the even order products don't make
it through the IFT. It also says vari-mu tubes are especially designed to
minimize 3rd order distortions, which do pass through the IFT to the
detector.
> If the peaks of an AM envelope are compressed on top and bottom,
> then that's odd order distortion of the whole wave form,
> but when one side of the compressed IF envelope is detected, the
> AF thd is mainly even order!
That sounds like a problem in the detector, not the IF amplifier. This
raises an interesting point that I will have to think about. The third
order distortions, which do make it through the IFT, may unbalance the
amplitudes of the upper and lower sidebands around the carrier, resulting
in the type of distortion you are talking about when an envelope detector
is used, but if this is the case, I don't think your idea of detecting
both the "ve- & ve+" sides of the wave form would fix the problem. A
synchronous detector would eliminate distortion due to unbalanced
sidebands, if indeed 3rd order distortion products can cause this. When I
get a chance in the next few days I will try to run through the math for
this and see if 3rd order distortion does unbalance the sidebands,
resulting in distortion when envelope detectors are used.
> > > > > In FM IF circuits, no need for balanced amps,
> > > > > because the linearity don't matter much, since the limiter output
> > > > > is just a 10.7 MHz signal which is devoid of amplitude changes,
> > > > > and distortion products of 10.7 MHz envelope can't
> > > > > appear in the audio, since the detector only
> > > > > detects variations in the distances between the
> > > > > up/downs of the 10.7 MHz waves.
> > > >
> > > > I'm not an expert in this, but I think this is a fallacy, and the
> > > > linearity of FM IF stages is actually quite important.
> > >
> > > Nope,
> >
> > I suggest you read up on your FM theory, there were a lot of good papers
> > published on this subject in the 1950s.
>
> But what about when the amp is under serious limiting?
> The IF amp is then saturated, and the current distortion is huge.
> The whole stage simply acts to convert the incomming IF wave a
> simple clipped sine wave, and the IF can be quite non linear in this
condition,
> when AF fidelity is then highest.
>
> FM radios represented nice big move away from the constrictions we
> need to carefully follow in AM recievers, where any FM would be deadly.
I put together a bibliography on this subject a while back, which I can
email to you if you are interested. I don't pretend to understand it all,
in fact I am far from completely understanding it. I think the problem
comes about because co channel signals, either other stations on the same
channel, or multipath from the desired station result in amplitude
modulation of the IF signal, which the limiters convert to phase
modulation. I'm not sure if I have the right grip on this, but one way to
deal with the problem seems to be to use a very wide band FM detector,
with no filtering between the limiter and detector. The other approach,
which I don't pretend to even begin to understand involves multiple
limiters with filters between which have specially chosen bandwidths,
which somehow allows the use of a narrower bandwidth FM detector, but the
filter bandwidths must be carefully chosen. The bottom line seems to be
if these factors aren't taken into consideration, then the suppression of
multipath, and co channel interference, will suffer, and the full
advantage of FM will not be realized. This just my possibly wrong, foggy
understanding of what some of the papers were trying to say.
John Byrns
fire...@sover.net (firedome) wrote:
> jby...@rcn.com (John Byrns) wrote in message
news:<jbyrns-1207...@216-80-74-223.d.enteract.com>...
> > In article <c534ce0f.0307...@posting.google.com>,
> > fire...@sover.net (firedome) wrote:
> >
> > > Who considered the S-R tuner the best commercial tuner ever made?...
> > > Audio Engineering (July 1953)..."there has never before been available
> > > a radio receiver which provided AM quality which approached FM" ...and
> > > Consumer Research Bulletin (March 1953)..."the first satisfactory
> > > detector circuit yet observed for high fidelity use in an AM tuner".
> >
> > Those are certainly some fantastic superlatives!
> >
> > > As for the circuit's provenance, I never said S-R invented it...
> >
> > And for my part I never said you said S-R invented it.
> >
> > > it was, as I said, an adaptation of the Selden -Smith 2 tube decector
> > > circuit (did any one else use 2 tube detectors?) found in the RDH3.
> >
> > I missed the part where you said that. The Ampex AM/FM simulcast stereo
> > tuner also used a similar "two tube" detector circuit. I forget the
> > actual Ampex model number of this tuner, but many people refer to it as
> > the "008" tuner, but that is not the actual model number.
>
> I mentioned the circuit in RDH3, not the inventors names... I'm going
> on what Will Rayment told me when I interviewed him for VTV, and these
> are his recollections of what happened 50 yrs ago.
Yes, I think the problem is that the circuit was invented more than 60
years ago, and nobody remembers the details. I have not been able to find
the circuit in the RDH3, and suspect that Will Rayment is misremembering
this. I suspect that Will is actually probably thinking of the RDH4,
which does show the circuit in an appendix of some printings. It would
also be interesting to know how S-R found out about the circuit, as I
don't think Water Selsted knows, although he did give S-R permission to
use the circuit when they asked him. I have to suspect that the circuit
probably became known by word of mouth in the California technical
community during the 1940s.
> McIntosh obviously
> thought highly of the S-R IF coils as they pursued it with diligence.
> I myself am not sure of the advantage of honeycomb construction, but
> McIntosh apparently thought there was one beyond marketing hype. I'll
> ask Sid Corderman if he knows when I interview him.
That would be an interesting question to ask, why do you say "McIntosh
apparently thought there was one beyond marketing hype", I would think the
opposite?
> As for SR-51s,
> there was one on eBay a few months ago, and I believe the SR-68 and
> later SR-1000 had essentially the same circuit.
Was the SR-51 the AM only version? I know they made a bewildering array
of versions, AM only, AM/FM, and with and without built in preamps.
Regards,
John Byrns
donut
74-203.d.enteract.com:
> Thanks, I will check the "oldradioparts" site, I never noticed that AES
> had any of the J.W. Miller IF Transformers that I am interested in.
On the AES site, hit Parts then Coils and coil forms.
The Miller NOS coils are listed near the bottom of the page. Several IF
transformers, though hard to tell if they are tube or transistor types.
Randy and/or Sherry
John Byrns wrote:
> A
> synchronous detector would eliminate distortion due to unbalanced
> sidebands, if indeed 3rd order distortion products can cause this.
Yeah but a really good synch detector can set you back major bucks - Are
they really worth it? I guess 30hz - 15Khz +/- 3db on AM is pretty good
(Sherwood Eng.'s is rated that) - but I wonder about distortion - there
is just something in my gut that says when you sub a local carrier for
the original - something isn't going to be as clean as original. But
I'll admit - I've always had a time understanding full pll synch
systems... --- I'm saving my pennies... but at $550+ --- it isn't a
high priority.
best regards...
John Byrns
> John Byrns wrote:
> > A
> > synchronous detector would eliminate distortion due to unbalanced
> > sidebands, if indeed 3rd order distortion products can cause this.
>
> Yeah but a really good synch detector can set you back major bucks - Are
> they really worth it? I guess 30hz - 15Khz +/- 3db on AM is pretty good
> (Sherwood Eng.'s is rated that) - but I wonder about distortion - there
> is just something in my gut that says when you sub a local carrier for
> the original - something isn't going to be as clean as original. But
> I'll admit - I've always had a time understanding full pll synch
> systems... --- I'm saving my pennies... but at $550+ --- it isn't a
> high priority.
Come on, today it only costs pennies, we do it all the time for FM stereo,
although in that case the carrier isn't transmitted, so your gut doesn't
have to worry. C-Quam AM stereo chips do it too, no big deal. Well, I
suppose it is not trivial doing it with tubes, and tube type FM stereo
used filters and doublers, rather than PLLs, but it would be doable.
Randy and/or Sherry
John Byrns wrote:
>
> Come on, today it only costs pennies, we do it all the time for FM stereo,
> although in that case the carrier isn't transmitted, so your gut doesn't
> have to worry. C-Quam AM stereo chips do it too, no big deal. Well, I
> suppose it is not trivial doing it with tubes, and tube type FM stereo
> used filters and doublers, rather than PLLs, but it would be doable.
Then how do they get $$$ for these?
http://www.sherweng.com/indepth.html
what's different here than say the C-Quam chips - or perhaps that is an
answer - before these chips $$$$ - if somebody figures how to adapt
these chips to accomplish similar performance???? If I could get that
kind of performance out of a cheap chip... hmmmmm....!
best regards..
Patrick Turner
John Byrns wrote:
It says enough to confirm that IF amps are one of the worst
sources of thd of a radio, because it is a voltage amplifier producing
considerable voltages.
There is no nfb involved, and the IF tube operates at a region where
introduced thd should be a worry for those who like real fidelity.
>
>
> > If the peaks of an AM envelope are compressed on top and bottom,
> > then that's odd order distortion of the whole wave form,
> > but when one side of the compressed IF envelope is detected, the
> > AF thd is mainly even order!
>
> That sounds like a problem in the detector, not the IF amplifier.
No, it isn't.
I have just outlined that the IF amp is capable of compessing the
waveforms, ie, adding distortions, and these deformations of the AF
modulation shapes are detected, and thus the IF tube has added muck to the AF
signal.
The ideal IF amp should be as linear as one can make it, period!,
if one wants maximum fidelity. And a balance has to be struck to
keep its voltage output lowish, but not too low to let noise start to rasie its ugly
head.
> This
> raises an interesting point that I will have to think about. The third
> order distortions, which do make it through the IFT, may unbalance the
> amplitudes of the upper and lower sidebands around the carrier, resulting
> in the type of distortion you are talking about when an envelope detector
> is used, but if this is the case, I don't think your idea of detecting
> both the "ve- & ve+" sides of the wave form would fix the problem.
I agree, once the odd distortions have mauled the IF envelope,
both top and bottom modulation shapes become distorted.
The thd cannot be undone, or filtered out.
The PP CF detector isn't really needed, because its thd with a tube like 12AT7,
or 6EJ7 or 6EJ7 will be very low, But the CF tube thd is cancelled
by PP action, so 12AU7 is quite OK to use.
> A
> synchronous detector would eliminate distortion due to unbalanced
> sidebands, if indeed 3rd order distortion products can cause this. When I
> get a chance in the next few days I will try to run through the math for
> this and see if 3rd order distortion does unbalance the sidebands,
> resulting in distortion when envelope detectors are used.
Good luck with the maths.
I prefer to build and measure things to find out the ultimate truth.
I have tried synchronous detection, and got bogged down
with spurious oscillations and strange noises.
But balanced synchronous detectros have less thd than SE types,
and the purist would go for these in preference to an SE type.
FM sets are prone to reflection of signals off buildings, thus cancelling
the recieved waves due to phase cancelling, reducing input,
and the IF signal goes out of the limited state, causing crackles, buzzes,
intermittent drop outs.
We all remember trying to get a pair of rabbit ear antennas on top
of a TV to give us a clear picture. There usually was only one postion which
seemed to work best, and that was the one where reflections were minimal, and
maximum
wanted signal came through.
I have had FM sets whose signals died when I've walked across the room.
We have a lot of separation between FM stations here.
Its impossible for two stations to be recieved at once.
I have never heard it, unless tuned well away from both stations,
then sometimes you get a noisy version of various stations.
But once tuned, with plenty of signal to drive the IF
into limiting, no problems.
> I'm not sure if I have the right grip on this, but one way to
> deal with the problem seems to be to use a very wide band FM detector,
> with no filtering between the limiter and detector.
The detector is wide band, the IF could be a wide range of F,
several MHz, and the FM wouls till be detected.
IF BW in FM sets is usually 300 kHz, and stations are not
permitted to use more than +/- 75 kHz deviation to
express the amplitude changes to their AF signals.
The sidebands produced by FM are far more complex than AM,
but at the end of the day, FM offers far better fidelity.
> The other approach,
> which I don't pretend to even begin to understand involves multiple
> limiters with filters between which have specially chosen bandwidths,
> which somehow allows the use of a narrower bandwidth FM detector, but the
> filter bandwidths must be carefully chosen.
Narrow bandwidth fm was used by the military at all sorts of carrier frequencies,
and not for hi-fi.
I have a 1959 army tranciever set using a single output tube, at 28 Mhz,
for local army communications. Beautifully made inside, very heavy,
and totally obsolete by today's standards.
Quite a few sets with 3 stage IF amps will go into limiting, ie,
the last two IF tubes are being driven into grid current, and well into
overload, so any input signal amplitude variations do not get through
to cause distortions to the audio detected bu the discriminator,
which is designed to respond to FM, not AM.
> The bottom line seems to be
> if these factors aren't taken into consideration, then the suppression of
> multipath, and co channel interference, will suffer, and the full
> advantage of FM will not be realized. This just my possibly wrong, foggy
> understanding of what some of the papers were trying to say.
Maybe you are right, and not everyone can get good FM reception, or TV.
I have line of sight to a 10,000 watt transmitter on top of a big tower on a large
hill
I see clearly about 7 Km away, so I get excellent TV and FM signals.
Ppl living in shadows of hills here get poor reception, and then some areas suffer
multipath, which interferes with the waves phase, causing unstable on-off
interferences.
I am not sure that such things can be all fixed by the quality of the receiver;
a lot depends on the transmitted signal.
Some suburbs have auxilliary transmitters at a different frequency,
placed elsewhere locally, to overcome reception problems in shadow areas.
Patrick Turner.
Patrick Turner
John Byrns wrote:
One could use a locked oscillator instead of PLL.
The early synchrodynes relied on this instead of PLL.
Patrick Turner.
Jeffrey D Angus
Randy and/or Sherry wrote:
>
> Then how do they get $$$ for these?
>
> http://www.sherweng.com/indepth.html
>
> what's different here than say the C-Quam chips - or perhaps that is an
> answer - before these chips $$$$ - if somebody figures how to adapt
> these chips to accomplish similar performance???? If I could get that
> kind of performance out of a cheap chip... hmmmmm....!
>
> best regards..
Audiophiles aren't the only one's to suffer from phoolishness.
Besides, it's like the early PC's way OVER priced, because they could.
Don't get me wrong, I'm sure the Sherwood stuff works every bit as
well as advertized.
So does Motorola. But even though the radios now use a computer
interface to add and subtract channels, the pricing is still based
on cost of channel elements.
Jeff
--
"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety." Benjamin Franklin
"A life lived in fear is a life half lived."
Tara Morice as Fran, from the movie "Strictly Ballroom"
Patrick Turner
Randy and/or Sherry wrote:
> John Byrns wrote:
>
> >
> > Come on, today it only costs pennies, we do it all the time for FM stereo,
> > although in that case the carrier isn't transmitted, so your gut doesn't
> > have to worry. C-Quam AM stereo chips do it too, no big deal. Well, I
> > suppose it is not trivial doing it with tubes, and tube type FM stereo
> > used filters and doublers, rather than PLLs, but it would be doable.
>
> Then how do they get $$$ for these?
>
> http://www.sherweng.com/indepth.html
>
> what's different here than say the C-Quam chips - or perhaps that is an
> answer - before these chips $$$$ - if somebody figures how to adapt
> these chips to accomplish similar performance???? If I could get that
> kind of performance out of a cheap chip... hmmmmm....!
>
> best regards..
> --
> randy guttery
Well the above website page gives glowing reports for their
own product, but exalted carrier has been around for a long time.
It is possible to add a 455kHz carrier to a normal IF signal,
abd have this carrier lock onto a sample of the 455 kHz IF.
This generally means an extra IF stage simply to pick out the wanted station
but with less BW.
Tube based locked oscillators are not that hard to build.
Usually, they use positive FB around a 455 kHz coil, and allow
some injection of a small voltage recovered from the transmitted carrier,
which triggeres the oscillator to oscillate at exactly the same F as the wanted
carrier.
As long as the incoming signal is over a certain low threshold,
and the wnated station doesn't use over say 90% modulation,
there should be enough carrier there to alaways synchronise the oscillator.
The oscillator signal will have no modulation in it.
It is then added to the IF signal, and you then get a
high peak to peak modulated IF signal, but with a small %
of modulation on it, so any diode detector decodes it OK.
There would still have to be good AVC, which often don't work very well
with short wave, because the signals are so low, and the tubes are
operating near maximum gain, but if amplified DC is fed back
the fading problem is much reduced, and all you here is the
variation in noise levels, increasing when the signal falls,
plus that strange phasey sound that SW stations have.
Digitising the whole signal recieved from the antenna, and
simply getting a PC to apply a program which analyses the signal digitally
is probably FAR better than any HF radio could be.
I've never tried this.
I still get a kick out of really good tube analog receivers.
Patrick Turner.
firedome
Ok, went back and looked at my interview notes...in late '51 before
Will's father passed away Walter Selsted had apparently developed some
kind of tweeter capable of super high frequencies (I'm not aware of
any details) in addition to the circuit found in RDH3 (Will may have
meant 4, maybe, but I wrote down 3 as that's what he recalled) that
was supposed to have a low distortion of around .35%. Selsted
contacted L.C. Rayment about SR using the circuit as they had a long
reputation for high quality products back to 1927, and arrangement was
made for S-R to use it. Two permeability tuned IF 6BA6 stages
terminated into the low distortion detector (1N48 & 12AU7). That's all
the detail I have on it...it was used in the SR 51 and 68 and 808
AM/FMs and also on the SR-58 AM only, and later, in the late 50s, in
the SR1000, which had provision for mpx and modern styling. Also in
the Cantilever Control Brentwood tuner (SR100) of the mid 50s...
according to my notes, McIntosh was interested in the
coils specifically because of their superior specs, and attempted to
have J-R winding of LA duplicate them, unsuccessfully. They also
copied the SR variable rumble filter that was designed specifically to
deal with the rumble from Garrard turntables. All of this info is
direct from Will Rayment, but as you said, filtered thru 60 yrs of
time, and who knows what all? Anyhow, a bit of contribution to the
history of those days.
best, Roger in NY
John Byrns
I pulled Sturley down off the bookshelf to check for more detail from the
reference given in the RDH4 for "Modulation Envelope Distortion". So far
I have only quickly scanned the first two of six pages Sturley devotes to
the subject, but it is clear what you say about "Modulation Envelope
Distortion" is correct, and second harmonic envelope components are
generated that would affect a synchronous detector as well as the simple
envelope detector. Sturley of course backs up the notion that the 2nd
order distortion products don't make it through the IFT, but the thing I
didn't realize was that third order distortion in the IF amplifier not
only produces sidebands around the carrier at three times the modulating
frequency, but also at twice the modulating frequency, and at the
modulating frequency. Sturley doesn't show the derivation of this, but I
now realize that it should have been immediately obvious to me simply from
the basics of third order distortion, without even going through the
equations.
Getting back to your original claims that a push pull IF amplifier would
reduce this distortion, and that rectifying both "-ve & +ve" sides of the
wave form in the detector would cancel second harmonic distortion, I can't
see how either of these would really help matters a great deal.
The push pull IF would only cancel the 2nd order distortions, which are
already not a problem due to the filtering action of the IFT, the only
distortion reducing benefit of push pull operation would seem to be that
with two tubes sharing the load, each could operate at half the power
otherwise required, with a consequent reduction in distortion, but this
has nothing to do with the cancellation of 2nd order distortion.
Similarly, rectifying both the "-ve" and "+ve" sides of the IF wave form,
and taking the difference, won't help because the damage has already been
done before the IFT feeding the detector stage, the "-ve" and "+ve" sides
of the IF wave form are mirror images of each other at the output of the
IFT, and the detector, even a synchronous detector, can't put it right
again.
So it seems we can't depend on much help from push pull operation of the
IF amplifier stage, or the detector, and must simply design the IF
amplifier tube, and supporting circuit, to be as free from 3rd, and
higher, order distortions as possible.
Regards,
John Byrns
In article <3F1226B6...@turneraudio.com.au>, Patrick Turner
<in...@turneraudio.com.au> wrote:
John Byrns
> Ok, went back and looked at my interview notes...in late '51 before
> Will's father passed away Walter Selsted had apparently developed some
> kind of tweeter capable of super high frequencies (I'm not aware of
> any details)
That was another Selsted & Smith collaboration, they published an article
on it in the January 1950 issue of "Audio Engineering" magazine.
> in addition to the circuit found in RDH3 (Will may have
> meant 4, maybe, but I wrote down 3 as that's what he recalled) that
> was supposed to have a low distortion of around .35%.
I can't find it in the RDH3, unfortunately it appears that they never
published an article on the "low distortion AM detector", the only thing I
have ever seen published about it is in the RDH4, and that doesn't delve
into its operation.
> Selsted contacted L.C. Rayment about SR using the circuit as they had a long
> reputation for high quality products back to 1927, and arrangement was
> made for S-R to use it.
If that is true, it would explain how S-R came to learn of the circuit, I
had come to the probably mistaken conclusion that it was the other way
around and S-R had contacted Walter about using it.
Did you interview Water Selsted about the detector?
> Two permeability tuned IF 6BA6 stages
> terminated into the low distortion detector (1N48 & 12AU7).
Interesting that S-R used a Germanium diode in some models, the SR-68 used
the two halves of a 6SN7, one half connected as a diode, and the other
half as a triode.
> That's all the detail I have on it...it was used in the SR 51 and 68 and
> 808 AM/FMs and also on the SR-58 AM only, and later, in the late 50s, in
> the SR1000, which had provision for mpx and modern styling. Also in
> the Cantilever Control Brentwood tuner (SR100) of the mid 50s...
I had what must have been the matching "Cantilever Control" amplifier,
used a pair of EL34s in the output, and had a "scratch" filter with a
variable cutoff point controlled by a two or three gang variable capacitor
like you would find in a radio, I think it may also have had a matching
rumble filter, whose cutoff frequency was controlled by a ganged pot.
> according to my notes, McIntosh was interested in the
> coils specifically because of their superior specs, and attempted to
> have J-R winding of LA duplicate them, unsuccessfully.
I wonder what characteristic it was about the S-R IF transformers, that
McIntosh was after?
> They also copied the SR variable rumble filter that was designed
> specifically to deal with the rumble from Garrard turntables.
Which McIntosh amplifier used the variable rumble filter, I don't remember
one, but then I am not that familiar with old McIntosh equipment?
> All of this info is direct from Will Rayment, but as you said, filtered
> thru 60 yrs of time, and who knows what all? Anyhow, a bit of contribution
> to the history of those days.
Patrick Turner
John Byrns wrote:
> I pulled Sturley down off the bookshelf to check for more detail from the
> reference given in the RDH4 for "Modulation Envelope Distortion". So far
> I have only quickly scanned the first two of six pages Sturley devotes to
> the subject, but it is clear what you say about "Modulation Envelope
> Distortion" is correct, and second harmonic envelope components are
> generated that would affect a synchronous detector as well as the simple
> envelope detector. Sturley of course backs up the notion that the 2nd
> order distortion products don't make it through the IFT, but the thing I
> didn't realize was that third order distortion in the IF amplifier not
> only produces sidebands around the carrier at three times the modulating
> frequency, but also at twice the modulating frequency, and at the
> modulating frequency. Sturley doesn't show the derivation of this, but I
> now realize that it should have been immediately obvious to me simply from
> the basics of third order distortion, without even going through the
> equations.
OK, say you have an IF of 455 kHz, and a AF tone of 1 kHz for the modulation.
The carrier of 455 kHz is present, and sidebands of 444 kHz, and 456 kHz.
Then if the envelope amplifier slightly compresses the envlope peaks,
as would be the case with added flattening 3rd harmonic distortion,
then the envelope then contains all the above fequencies, plus
additional sidebands at 452 kHz and 458 kHz, and this extra muck
makes it through the IF tranny BW, and its all detected loud and clear
and can be measured as distortion to the AF waves from the detector.
>
>
> Getting back to your original claims that a push pull IF amplifier would
> reduce this distortion, and that rectifying both "-ve & +ve" sides of the
> wave form in the detector would cancel second harmonic distortion, I can't
> see how either of these would really help matters a great deal.
I agree.
But rectifying two phases with two tubes increases the dynamic range of the
detector by 6 dB.
The detector needs to be in reciept of signals from a low distortion
voltage amp if we want the receiver to be low distortion, as a whole.
If we want the best AM that tubes can provide, then a balanced IF
amp makes sense, and this could produce up to maybe 100 vpeak to peak envelope
voltage,
so one has to think carefully about the following detector voltage capability.
Once such an enormous dynamic range is established, the thd should be
low at a few volts of output.
>
>
> The push pull IF would only cancel the 2nd order distortions, which are
> already not a problem due to the filtering action of the IFT, the only
> distortion reducing benefit of push pull operation would seem to be that
> with two tubes sharing the load, each could operate at half the power
> otherwise required, with a consequent reduction in distortion, but this
> has nothing to do with the cancellation of 2nd order distortion.
But if there is 2H distortion in a tube amplifying an enevelope,
does this not create sidebands at 453 kHz, and 457 kHz, in the above case,
and would they not proceed throught the IFT, and get detected?
I haven't measured this phenomena for awhile, so I just don't know.
>
> Similarly, rectifying both the "-ve" and "+ve" sides of the IF wave form,
> and taking the difference, won't help because the damage has already been
> done before the IFT feeding the detector stage, the "-ve" and "+ve" sides
> of the IF wave form are mirror images of each other at the output of the
> IFT, and the detector, even a synchronous detector, can't put it right
> again.
Exactly, but the PP detector has 6 dB more dynamic headroom, so
shouls operate with less thd.
In practice a 12AT7 cathode follower with both halves paralleled
operating with an Ea of 200v, and 6 mA of current should cope
with the unbalanced secondary output from an IFT driven
by a balanced PP IF amp.
>
>
> So it seems we can't depend on much help from push pull operation of the
> IF amplifier stage, or the detector, and must simply design the IF
> amplifier tube, and supporting circuit, to be as free from 3rd, and
> higher, order distortions as possible.
A PP IF amp will achieve such an outcome, easily, because of the
increased dynamic range.
The PP thd is all mainly 3H, but it usually isn't any more than twice what it is
in a SE amp using the same tube, and pentodes when carefully set up
don't have enormous 3H at moderate voltage levels, although
they quickly start to show an awful range of distortion products
once past about 10v of output.
In the radio I have built, I use a 6BX6, with about 6 dB current FB,
from an unbypassed Rk, and this amp is adequately linear for me,
but a balanced amp doing the same thing probably would have 1/4 the thd
at the same voltage output.
I control the voltage output with AVC applied only to the twin vari-mu triode
RF amp ahead of the mixer.
There is very little voltage produced by the triode input stage, so
its linearity wouldn't benefit much with a balanced circuit,
and the mixer also works with fixed bias, a high oscillator current change,
but small RF input signal, so it is linear.
With short wave, allowing the final IF amp to have a higher dynamic range should
better the SNR, But then the approach is different, with an extra IF stage needed,
maybe double conversion, mechanical or crystal filters, depending how far we want
to go,
but hi-fi can't be our main aim with SW reception.
I will build an extra IF tube into my set one day, and measure before and after.
I envisage a DC feed to the tubes using an RF choke with CT, then
a normal IFT could be used, a sort of parafeed PP IF amp.
Patrick Turner.
Brian
> reference given in the RDH4 for "Modulation Envelope Distortion". So far
When you get done reading, John, I'd be interested to know some
typical distortion figures. I'll bet they're signficant only for very
strong signals. I know that some receivers attenuate the AVC to the
last IF stage to minimize envelope distortion since that's where the
highest signal levels occur. Still, my guess is that this source of
distortion pales in comparison to that due to low modulation
acceptance.
I'm restoring a Hallicrafters S-76 boatanchor. Stock, it was the worst
receiver I've ever encountered for modulation acceptance, clipping on
the downward side at 53% modulation or more.
Brian
John Byrns
<in...@turneraudio.com.au> wrote:
The thing that I missed, that Sturley points out, and I would have
eventually seen once I got around to the math, but which should have been
obvious to me with only a moments thought, is that 3rd order non linearity
in the IF amplifier not only produces sidebands at 452 kHz, 454 kHz, 456
kHz, and 458 kHz as you mention, but also at 453 kHz and 457 kHz, creating
the "2H audio distortion you have observed. This is why push pull IF
amplifiers can't eliminate this "2H" audio distortion, because it is the
result of a third order effect in the IF amplifier, which isn't canceled
by the push pull action that cancels even order non linearity. To put it
in other words, the 3rd order distortions in the IF amplifier
symmetrically squash the IF wave form, but the result of that is that the
envelope is squashed on only the positive modulation peaks, creating a
second order distortion in the recovered audio.
> > The push pull IF would only cancel the 2nd order distortions, which are
> > already not a problem due to the filtering action of the IFT, the only
> > distortion reducing benefit of push pull operation would seem to be that
> > with two tubes sharing the load, each could operate at half the power
> > otherwise required, with a consequent reduction in distortion, but this
> > has nothing to do with the cancellation of 2nd order distortion.
>
> But if there is 2H distortion in a tube amplifying an enevelope,
> does this not create sidebands at 453 kHz, and 457 kHz, in the above case,
> and would they not proceed throught the IFT, and get detected?
> I haven't measured this phenomena for awhile, so I just don't know.
No, the second order distortion in the tube creates sidebands around 910
kHz, or twice the IF frequency, at 908 kHz, and 912 kHz, and these get
blocked by the IFT, just as the DC and audio frequency components produced
by 2nd order distortion.
This has all been very useful, because I now realize that to minimize
audio distortion you want to avoid aperiodic couplings between the last IF
amplifier stage, or RF amplifier stage in a TRF receiver, and the
detector. This is a rule that is violated by some well known TRF
receivers, including the Western Electric No. 10A, the Weeden, and the
J.W. Miller 570.
Regards,
John Byrns
donut
true hi-fidelity AM.
How many stages of RF amplification would a TRF need to equal the
sensitivity of an AA5?
I know there's a problem with selectivity but the more tuned circuits, the
better that would get.
Would 5 tank circuits and 5 RF amplifier stages, all operating near unity
gain (except for the first one) be sufficient?
Would each RF stage add some distortion of it's own?
I ask this question because I can get a 5 gang variable cap very
inexpensively.
Fred Nachbaur
There may be more knowledgable folk who will differ, but it seems to me
that it's immaterial whether you amplify the RF directly, or after
conversion to a fixed intermediate frequency. The only real difference
is that in a superhet, tracking difficulties are greatly reduced because
you're dealing with only one frequency.
In principle, at least, a TRF with the same number of amplifier stages
and the same number of tuned circuits with the same Q will perform as
well as a superhet. But there's that tracking problem again...
What's more, as John Byrn pointed out in an earlier post, an IF
transformer's coupling can be tailored for a reasonably
square-shouldered response curve. However, this only works properly at a
single frequency, so your TRF's response will vary across the band. That
may not be an issue if you only plan to listen to a single station, in
which case you could, in principle, tailor your tuned circuits exactly
for that frequency. You'd essentially have a superhet without the "het".
;-) (I.e. just the RF strip, detector, and amplifier, with no heterodyne
conversion necessary.)
Five tuned stages would be an overkill, IMO, even at low gain. Don't
forget that high-Q tuned circuits have an apparent voltage gain of their
own, so even low-mu triodes can have gains as TRF amplifiers many times
the published amplification factor (mu). You have to be careful to
shield each stage, or you'll end up with a string of TPTG (tuned-plate,
tuned-grid) oscillators.
And yes, each stage can introduce its own distortion, whether it be RF,
IF, or audio. John Byrn's and Patrick Turner's posts on the subject
speak volumes.
Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+
Brenda Ann
"Fred Nachbaur" <fnac...@netscape.net> wrote in message
news:KUMQa.9837$xn5.1...@news0.telusplanet.net...
>
> There may be more knowledgable folk who will differ, but it seems to me
> that it's immaterial whether you amplify the RF directly, or after
> conversion to a fixed intermediate frequency. The only real difference
> is that in a superhet, tracking difficulties are greatly reduced because
> you're dealing with only one frequency.
>
The biggest problem with TRF isn't the varying bandwidth, or even the
tracking.. but the tendency for feedback. The more stages you have at the
same frequency, the more likely it is that you will have feedback problems.
This is where the characteristic squeal and swishing sounds you hear
associated with very old radios come from. TRF's would break into
oscillation on a whim, unless each stage was very well shielded from all the
others. IIRC, this is how regenerative receivers were discovered/invented.
Patrick Turner
Brenda Ann wrote:
Yes, instability is a problem with TRF, and if you had 5 tuned circuits,
with 4 tubes between them, then at least you wouldn't need high transconductance
tubes.
But an IF amp in any radio one might build can be instable, especially a two
stage type.
The answer is very short connecting wires, and well thought out layout, and
screening.
Quite a few communication receivers had cast and machined aluminium chassis,
with
"pockets" for each RF stage, none of this cheap pressed metal garbage seen on
consumer grade radios. The 22 tube Racal is such an example.
The DIYer can achieve excellent results, but it all takes time, time, and more
time.
If you had a single LC Q of 100 at 1,000 kHz, then the BW is 10 kc, so with
4 more other circuits after this, the effective Q would be higher, and BW lot
lower,
so expect only 3kHz RF BW, so 1.5 kHz AF bandwidth.
The skirt steepness away from resonance will be 5th order,
or 30 dB/octave, which would just give enough rejection of a strong station
at 50 kHz above or below the wanted station at 1,000 kHz.
The receiver will be selective, but the sound won't be hi-fi.
For hi-fi, we want the characteristic of a critically coupled IF transformer,
which only has to deal with one F, and the magnetic coupling which is determined
by the
distance between the coils will determine the shape of the bandpass filter.
A single tuned LC gives a pointy to to the nose shape,
while the transformer gives either a really sharp nosed response,
or flat or double peaked, when the coils are close.
The tuned transformer still gives skirt roll off shapes at an ultimate second
order slope.
I suggest ppl who have doubt about this should go back to the old books,
and do some real study, to save us from having to regurgitate what is in those
textbooks.
So off your bums, away from the PC and start reading!!!!
Not in too many books is the idea of using about 2.4 Mhz as the IF frequency.
If the Q of the LC was 100, then BW with a tuned circuit could be
24 kHz, so with a lot of tuned circuits, you'd still get a fair RF BW.
But IFT with 2.4 Mhz and with spaced coils for a critically coupled flat
response,
the BW would be quite wide, and 3 such IFTs could be used
to make a BW of at least 20 kHz, giving 10 kHz AF bandwidth,
and the six tuned IF LC units would give 6th order ultimate slope of the skirt,
so selectivity for local stations would be fine.
The use of a PP IF amp has been criticised roundly by the proponents
of the status quo, for all sorts of reasons, the main one being that some
of the distortion don't matter, its obstructed/cancelled by an IFT.
I think many of you are all missing my point, which is that AM should be as
hi-fi as possible,
as much of what is transmitted is now hi-fi at the transmitter,
ie, low thd, but slightly restricted in AF BW, but very listenable indeed,
if ONLY we had a decent reciever that didn't dump 7% of thd into the signal,
and further restrict the BW to less than two bleeding kHz!!!!
This is typical of many old radios.
It is NORMAL for most solid state AM reciever sections that have been foistered
onto the public for the last 40 years.
FM is king, and AM is the poor relation.
To address this problem I suggest a PP IF amp isn't as crazy /wacky as you
think.
Its simply an application of the well known fact that a PP amp
using two tubes produces a far cleaner result than one SE tube,
for the amount of power produced by one tube on its own.
The dynamic range is potentially increased 6dB, or doubled, but
what I suggest is worth having is the simple absence of distortion.
In my home designed superhet, I have a sharp cut off pentode,
with 6 dB of current FB, with cathode bias, which stays fixed, and no AVC
applied to the IF amp, and I hear the difference between this and most other
radios,
which are just historical junk by comparison, ie, bean counter designed
consumer electronics, lowest common denominator stuff.
I do like them as works of art, but good performers they are not.
For example, I have had a 1939? STC MW/SW receiver here for a week to
repair. I did the usual, replace ALL the caps; even the paper ones rated at
1,500 volts
were leaky. Modern poly caps made after 1960 are far better.
The tubes all tesed fine, and I finally got all the switches to work, and then I
aligned and tuned it, and sure, its a great perfomer, for its time, with the
peaking in the speaker AF response at between 1 and 4 kHz partially compensating
for the 2 kHz of normal AF response I measured at the detector output.
Not all tube superhets have very wide AF bandwidth.
But with such a small RF BW, it is not too bad a performer on SW, for a
set without an RF stage.
With a long wire antenna, it even picks up amateurs OK, except
that only a few of them use AM, and most use SSB, and there is no BFO,
and the tuning rate is way to fast.
I have a Radiola from 1957, with an RF stage, and it was no better,
and it to gave that "melow sound" resonating woodwork, combined with
no treble, or treble simply replaced by distortion products, when turned up a
bit.
By means of strapping 100k resistors across each LC circuit in the IF,
I was able to lower the Q, and get about 7 kHz max, but its only good for
local stations. I revised the AVC to give the IF tube a bit of a break.....
I added the CF detector circuit, and with a PP audio amp, with a better circuit
than
Radiola ever had, it now sounds like a radio should, crisp treble,
creamy, rich bass, and a pleasure to listen to. It has about 11 tubes.
I have another of these chassis, (which originally were bought by only
wealthy folks,) and I eventually will upgrade it to
being a lot better for SW, if I can only work out how to align the
tracking and coil gain since there are six SW bands.
I won't bother with a PP IF, because the signals with SW are usually small,
and N&D from atmospheric conditions make it useless to get a better sound.
I doubt it is practical to make a simple AM radio which can wear all types of
hats,
ie, be great at both DX and local MW stations, and also be good at SW.
So In the fullness of time, I will have a seperate set for local MW,
and another for SW, and I don't care about MW DX, because
there is so much programme crap, and then there is the noise and fade outs.
Patrick Turner.
Nothing40
Hey Brenda Ann,Nice to see you hangin around here. :-)
Brenda Ann
"Nothing40" <noth...@yahoo.com> wrote in message
news:e8d7125c.0307...@posting.google.com...
> Hey Brenda Ann,Nice to see you hangin around here. :-)
Thanks, at least we aren't getting 300 idiotic posts a day in here. (yet)
:)
firedome
> In article <c534ce0f.03071...@posting.google.com>,
> fire...@sover.net (firedome) wrote:
>
> > Ok, went back and looked at my interview notes...in late '51 before
> > Will's father passed away Walter Selsted had apparently developed some
> > kind of tweeter capable of super high frequencies (I'm not aware of
> > any details)
>
> That was another Selsted & Smith collaboration, they published an article
> on it in the January 1950 issue of "Audio Engineering" magazine.
>
> > in addition to the circuit found in RDH3 (Will may have
> > meant 4, maybe, but I wrote down 3 as that's what he recalled) that
> > was supposed to have a low distortion of around .35%.
>
> I can't find it in the RDH3, unfortunately it appears that they never
> published an article on the "low distortion AM detector", the only thing I
> have ever seen published about it is in the RDH4, and that doesn't delve
> into its operation.
>
> > Selsted contacted L.C. Rayment about SR using the circuit as they had a long
> > reputation for high quality products back to 1927, and arrangement was
> > made for S-R to use it.
>
> If that is true, it would explain how S-R came to learn of the circuit, I
> had come to the probably mistaken conclusion that it was the other way
> around and S-R had contacted Walter about using it.
That's the info I got, I think that Selden had known LC from earlier
days in the 30s when SR was heavily into making military radio stuff
for the US Navy...didn't talk to Selden,didn't know he was still
around, my emphasis and interest was more slanted to the post-war S-R
amps and FM as AM was largely waning by then ...I have the separate
Cantilever Control basic mono amp (2 x 6L6) and tuner-preamp, I've
been looking for one of the integrated mono EL-34 amps like the one
you had, if you know of any please let me know...I have an SR-5100
El-34 stereo amp and it is excellent, along with the matching SR-2000
stereo preamp. S-R is little known, unfortunately, as they made
excallent products.
> Did you interview Water Selsted about the detector?
>
> > Two permeability tuned IF 6BA6 stages
> > terminated into the low distortion detector (1N48 & 12AU7).
>
> Interesting that S-R used a Germanium diode in some models, the SR-68 used
> the two halves of a 6SN7, one half connected as a diode, and the other
> half as a triode.
>
> > That's all the detail I have on it...it was used in the SR 51 and 68 and
> > 808 AM/FMs and also on the SR-58 AM only, and later, in the late 50s, in
> > the SR1000, which had provision for mpx and modern styling. Also in
> > the Cantilever Control Brentwood tuner (SR100) of the mid 50s...
>
> I had what must have been the matching "Cantilever Control" amplifier,
> used a pair of EL34s in the output, and had a "scratch" filter with a
> variable cutoff point controlled by a two or three gang variable capacitor
> like you would find in a radio, I think it may also have had a matching
> rumble filter, whose cutoff frequency was controlled by a ganged pot.
>
> > according to my notes, McIntosh was interested in the
> > coils specifically because of their superior specs, and attempted to
> > have J-R winding of LA duplicate them, unsuccessfully.
>
> I wonder what characteristic it was about the S-R IF transformers, that
> McIntosh was after?
Will mentioned 3 stages instead of 2, but I'm not sure what that
pertained to, as I am no expert on this stuff, more of a historian
than technician. It was clear that Mac was interested in the IF
performance aspect however.
> > They also copied the SR variable rumble filter that was designed
> > specifically to deal with the rumble from Garrard turntables.
>
> Which McIntosh amplifier used the variable rumble filter, I don't remember
> one, but then I am not that familiar with old McIntosh equipment?
Perhaps the C-20? I will check, I haven't gotten into the upcoming
Corderman/Painchaud/Rowley interview yet, so not up to speed on Mac
specs...
best
Roger in NY
John Byrns
> OK - the IF distortion was one reason that the TRF was favored for really
> true hi-fidelity AM.
RF amplifiers have exactly the same distortion problem as IF amplifiers,
of course the TRF eliminates the mixer/oscillator and all their associated
problems.
> How many stages of RF amplification would a TRF need to equal the
> sensitivity of an AA5?
Two well designed RF stages ought to do it, the gain per stage probably
won't be quite as great as an IF amplifier stage, but this is compensated
by the fact that the conversion gain of the converter stage in the AA5 is
not that high.
> I know there's a problem with selectivity but the more tuned circuits, the
> better that would get.
>
> Would 5 tank circuits and 5 RF amplifier stages, all operating near unity
> gain (except for the first one) be sufficient?
5 tank circuits would be good, but I suggest using only two RF amplifier
stages, as this should be able to equal the gain of the AA5, and 5 stages
would be a nightmare to layout so you don't get unwanted feedback and
oscillation.
Just cascading 5 single tuned tank circuits is going to give you a filter
with a very poor shape factor. I suggest you use a single tuned circuit
for the antenna, and a pair of double tuned circuits, one between the two
RF amplifier stages, and the second between the second RF amplifier stage
and the detector. This will give a better shape factor. Add some series
resistance to the tank circuits, and keep the source and load resistances
high, so that the Q increases with frequency, and then on the two double
tuned circuits use the coupling scheme used by the Western Electric No.
10-A, Weeden, and J.W. Miller TRF tuners, so that the coupling coefficient
varies inversely with frequency, giving you a nice approximation of a
constant bandwidth at both the high and low ends of the AM broadcast
band. J.W. Miller called this scheme "negative mutual coupling". Don't
make the mistake those three radios did of using aperiodic coupling
between the detector, my discussion with Patrick showed why that is a bad
idea. And even more important, unless you live way out in the sticks,
don't use an aperiodic antenna coupling stage as Miller did, as this will
let the powerful nearby stations into the first RF amplifier stage,
creating all sorts of havoc.
> Would each RF stage add some distortion of it's own?
Yes, the distortion situation is identical with that of the IF amplifiers.
> I ask this question because I can get a 5 gang variable cap very
> inexpensively.
Go for it then.
Regards,
John Byrns
John Byrns
<in...@turneraudio.com.au> wrote:
> To address this problem I suggest a PP IF amp isn't as crazy /wacky as you
> think.
> Its simply an application of the well known fact that a PP amp
> using two tubes produces a far cleaner result than one SE tube,
> for the amount of power produced by one tube on its own.
> The dynamic range is potentially increased 6dB, or doubled, but
> what I suggest is worth having is the simple absence of distortion.
I don't understand where you are getting this 6 dB figure from, I would
think it would be closer to 3 dB? Keep in mind that the distortion
reducing feature of PP amps is largely lost in this application, so the
principal advantage of the PP circuit is a doubling of the power available
for driving the detector.
Regards,
John Byrns
Patrick Turner
John Byrns wrote:
> In article <3F13D137...@turneraudio.com.au>, Patrick Turner
> <in...@turneraudio.com.au> wrote:
>
> > To address this problem I suggest a PP IF amp isn't as crazy /wacky as you
> > think.
> > Its simply an application of the well known fact that a PP amp
> > using two tubes produces a far cleaner result than one SE tube,
> > for the amount of power produced by one tube on its own.
> > The dynamic range is potentially increased 6dB, or doubled, but
> > what I suggest is worth having is the simple absence of distortion.
>
> I don't understand where you are getting this 6 dB figure from, I would
> think it would be closer to 3 dB?
Well, each half of the PP circuit can produce the same output
voltage, but oppositely phased, so potentially, there is twice the voltage
produced. Elementary, dear Watson.
The load a-a must be twice the single tube load.
But even if this isn't the case with a parafeed set up using a standard IFT,
there is still second harmonic current cancelling in the tubes, and it has to
give
lower distortion.
> Keep in mind that the distortion
> reducing feature of PP amps is largely lost in this application,
I am not so sure, I still think the PP would be advantageous,
and it needs to be built and measured, before deciding if its better or not.
> so the
> principal advantage of the PP circuit is a doubling of the power available
> for driving the detector.
Well, twice the voltage output, if possible, so the detector should be made
to have as high a voltage capability as possible, able to detect 50vrms.
Then when only 2 vrms is wanted, expect thd to be negligible.
With high level signals, the snr should be excellent.
Usually, most local station AM radios are quire OK regarding snr,
its not until you get up into the high SW bands that one might think about
triode mixers, et all, for low noise, perhaps a fet input stage.
There is no other point to PP IF amps, and a CF detector, then with a CF buffer,
other than chasing fidelity.
Its easy to make a fair sounding radio with the standard fare, mixer, vari mu IF
amp,
diode detector, then two audio stages with no FB.
Fair is all it will be.
One of these days, I must try the PP IF stage, but with IF = 2.4MHz,
and two stages, the first can be a vari mu stage, since its signal is very low.
making the IFTs and oscilator coils should be quite easy; less turns,
and maybe not even need litz wire.
I recall someone even tried a higher IF frequency....
For MW, low transconductance pentodes would be best, since too much gain in
two IF stages would be a hassle.
There must be 1,001 ways of building a radio.
Patrick Turner.
donut
news:3F13D137...@turneraudio.com.au:
> I suggest ppl who have doubt about this should go back to the old
> books, and do some real study, to save us from having to regurgitate
> what is in those textbooks.
> So off your bums, away from the PC and start reading!!!!
1) Some of us don't have access to the old books.
2) We are helping you keep your thought processes active. ;)
firedome
> > was, as I said, an adaptation of the Selden -Smith 2 tube decector
> > circuit (did any one else use 2 tube detectors?) found in the RDH3.
> > to use it in a commercial product AFAIK.
>
> I tried to email you about some information that I received from Walter
> Selsted on the "two tube" detector, but your email address bounces.
> Perhaps you already talked to Walter, did you interview him for your
> article on the S-R tuner?
Oh, I changed ISP... it's fire...@westelcom.com. I wrote earlier
today re: Walter, I didn't know he was still with us. You might want
to talk to Will Rayment, I'm sure he'd be happy to oblige, eMail me
for contact info... An article on this AM circuit might make a good
VTV submission, Charlie would welcome a well written piece on it I'd
imagine...
Roger in NY
John Byrns
(Brian) wrote:
> > I pulled Sturley down off the bookshelf to check for more detail from the
> > reference given in the RDH4 for "Modulation Envelope Distortion". So far
>
>
> When you get done reading, John, I'd be interested to know some
> typical distortion figures.
OK, I quickly scanned ahead and I see mentions of things like 5%
distortion, which I believe is a typical radio design goal, so he
presumably is shooting for something like that. It looks like he
discusses how to calculate the distortion from the tube characteristic
curves, but the real meat is probably in the references he gives at the
end of the chapter, hopefully he gives enough in the book to work it out.
I see one point where he discusses calculating the second harmonic
envelope distortion by operating the tube as an audio amplifier, and
measuring the third harmonic distortion of a sine wave, this fits right in
with the discussion Patrick and I have been having.
> I'll bet they're signficant only for very strong signals.
Clearly the IF amplifier distortion will go down at lower signal levels,
and of course diode detector distortion tends to rise at low signal
levels. Diode detector distortion seems rather minor until the modulation
gets high enough to get into negative peak clipping problems.
> I know that some receivers attenuate the AVC to the
> last IF stage to minimize envelope distortion since that's where the
> highest signal levels occur.
I will have to actually read the section from Sturley to see what he is
really saying but on my quick scan, it looked like he said the distortion
actually decreases as the negative bias on the vari-mu IF amplifier tube
becomes more negative, but that is in relation to a fixed input voltage to
the grid. Clearly in a real radio if the AGC is trying to maintain a
constant voltage at the detector, then the grid drive is going to have to
increase, and my question would be how does the distortion compare then.
To ask the question another way, for a given output from the tube is the
distortion lower with high negative bias and signal input to the grid, or
with lower negative bias and signal input? It will be interesting to see
if he addresses the issue from that perspective.
> Still, my guess is that this source of
> distortion pales in comparison to that due to low modulation
> acceptance.
But the distortion due to "low modulation acceptance" increases abruptly
at some point as the modulation level increases, while the amplitude of
the distortion caused by the IF presumably follows the second power of the
modulation level, and the third power of the signal level.
Regards,
John Byrns
John Byrns
> Oh, I changed ISP... it's fire...@westelcom.com.
I tried the westelcom ISP address that I had from when you emailed me late
last year, and it bounced, but the user name was "fireflite", so I assume
that was the problem. Is there an article lurking in this
firedome/fireflite business?
> I wrote earlier today re: Walter, I didn't know he was still with us.
Yes, I assume he is, he was back in January.
> You might want
> to talk to Will Rayment, I'm sure he'd be happy to oblige, eMail me
> for contact info...
OK, thanks, but first I want to see if I can get directly in touch with
Water Selsted, and ask him if he remembers how S-R became aware of the
detector, as that was a question I didn't think of originally, and it is
more interesting now that it appears he never published anything on the
detector.
> An article on this AM circuit might make a good
> VTV submission, Charlie would welcome a well written piece on it I'd
> imagine...
By "AM circuit" I assume you mean the entire tuner, not just the
detector? I am most interested in the history of the detector itself, and
while I have seen it referred to as a diode detector followed by a cathode
follower, I don't feel that is an accurate description, as the cathode
follower is operating in a nonlinear mode. The Selsted Smith detector
appears to me to be a slight variation on the so called "infinite
impedance", or reflex detector, which was developed by RCA. The intended
operation of the Selsted Smith detector is somewhat obscure, I have never
seen a technical article that discusses its design and operation. I have
been trying to run down some definite information on the RCA "infinite
impedance", or reflex detector, presumably there is an RCA licensee
Laboratories Bulletin describing its design and operation, but finding a
source for the RCA licensee Bulletins from the mid 1930's is a problem.
As a result of this exchange I did stumble across a reference to a three
page 1939 Wireless World article on the "infinite impedance", or reflex
detector, in addition to the 1937 article I already have. I will have to
try and run down a copy of the 1939 Wireless World article, is anyone out
there a Wireless World collector, that might have a copy of the August 3,
1939 issue of Wireless World? Information on these old high performance
AM envelope detectors is not the easiest thing to find.
Patrick Turner
donut wrote:
OK, then do some MORE research on established websites,
if you can't find the old books, which are carried by many second hand
bookstores,
some of which list their books on their websites.
I wish I had the time to restate what's in the text books all day,
but I just don't, and regurgitation would send me barmy.
And yet I don't like to see ppl continue in their ignorance,
so you'll just have to put up with a curt suggestion from time to time.
I like to try to enlighten folks about what might NOT be in the books or
so obvious.
I enjoy helping, and being there for ppl who try to help themselves,
especially if they have done some serious research they from the available
shelves full of info.
No offence meant,
And good luck with your projects, they usually take time, more time, and
tomorrow, before they amount to much.
Years later, when you see where you have been, your efforts were not in
vain..
Patrick Turner.
Patrick Turner
John Byrns wrote:
The S&S detector has the IFT secondary grounded at one end of the coil,
and the live end connected to a tube diode in series with 250k
to ground.
So the load seen by the IF amp tube is a part only of the reflected load of a
diode
turning on through an R, and in normal operation at a few volts, the diode
conducts continuously,
because the modulation is not always much more than 60%.
Thus a load of about 250k+ is on the IFT, and driving tube, and this
has a very small effect on the distortion, and far less effect than
if there was a cap to charge as is the case with most normal detectors.
The voltage at the top of the 250k is sent to a grid of a CF, and at the CF
cathode, there is a 15k to ground, with some current flow, since the carrier
input voltage means the average level is at a couple of volts.
There is a 150pF across the 15k, and this is charged positively by the 455 kHz
charge
peaks, just like a mains halfwave rectifier.
the 15k tries to discharge the cap, so the voltage at the CF cathode is like
the ripple voltage of a power rectifier, sawtooth shaped.
The AF modulation causes the AF voltage to vary at the cathode, just like a
changing
mains voltage will give a changing DC voltage at the output of a powe
rectifier.
After the cathode detection, the S&S detected signal is fed to an output to an
audio amp,
via a 0.1 uF and 1M bias resistor.
That's what is shown on page 1,495 of RDH4.
BUT, some 455 kHz ripple voltage will remain in this circuit, so it should be
filtered
out further with say an extra LPF RC filter, perhaps 50k, then another 150 pF,
which would have a -3dB pole at 20 kHz, allowing a cleaner audio signal to
get to the AF amp.
The thd quoted for this detector is 0.4% at 4 Khz of AM modulation, at 80%
modulation.
I think it can be improved easily, with the diode placed AFTER the CF, not
before,
to remove all loading effects on the IF amp, whose thd is NOT included in
the graph for detector distortion ( we can assume ), in RDH4.
Then the CF can be a 12AT7, so in CF, its thd will be far less than a 12AU7.
Then the idle current operation of the CF should be with say 1.5 mA flowing
down via
a 30k RL, or more, with the IF secondary biased up at about +40v.
The diode can be a germanium type, and set up with 0.5mA of current from a
R to some -Ve voltage, and this eliminates large variations in
diode forward voltage variations, even at low signal levels,
so the diode then contributes SFA thd since it spends its life turned on all
the time.
The value of C charged by the diode is determined by the idle current flowing
in the
diode bias resistor, and should be determined experimentally.
One of these days I'll publish my notes.
Expect detector to have less than 0.1% thd at any modulation F,
and up to 100% modulation.
I wonder what Walter might say about all this.
Patrick Turner.
firedome
> In article <c534ce0f.03071...@posting.google.com>,
> fire...@sover.net (firedome) wrote:
>
> > Oh, I changed ISP... it's fire...@westelcom.com.
>
> I tried the westelcom ISP address that I had from when you emailed me late
> last year, and it bounced, but the user name was "fireflite", so I assume
> that was the problem. Is there an article lurking in this
> firedome/fireflite business?
Only if you're into vintage DeSotos...another hobby...luckily,
firedome can apply equally to a certain vacuum state amplification
device...
> > I wrote earlier today re: Walter, I didn't know he was still with us.
>
> Yes, I assume he is, he was back in January.
Jeez, he must be up there agewise...interview him while you can! This
old audio history needs to be preserved...I'm up to my ears with Mc,
Pilot, and
Stromberg-Carlson on my plate for the forseeable future, a couple
years at least.
> > You might want
> > to talk to Will Rayment, I'm sure he'd be happy to oblige, eMail me
> > for contact info...
>
> OK, thanks, but first I want to see if I can get directly in touch with
> Water Selsted, and ask him if he remembers how S-R became aware of the
> detector, as that was a question I didn't think of originally, and it is
> more interesting now that it appears he never published anything on the
> detector.
>
> > An article on this AM circuit might make a good
> > VTV submission, Charlie would welcome a well written piece on it I'd
> > imagine...
>
> By "AM circuit" I assume you mean the entire tuner, not just the
> detector?
I think the detector itself and evolution of dector design in general
would make an interesting article...
John Byrns
<in...@turneraudio.com.au> wrote:
> The S&S detector has the IFT secondary grounded at one end of the coil,
> and the live end connected to a tube diode in series with 250k
> to ground.
> So the load seen by the IF amp tube is a part only of the reflected load of a
> diode
> turning on through an R, and in normal operation at a few volts, the diode
> conducts continuously,
> because the modulation is not always much more than 60%.
If the diode "conducts continuously", why is it even in the circuit?
> Thus a load of about 250k+ is on the IFT, and driving tube, and this
> has a very small effect on the distortion, and far less effect than
> if there was a cap to charge as is the case with most normal detectors.
>
> The voltage at the top of the 250k is sent to a grid of a CF, and at the CF
> cathode, there is a 15k to ground, with some current flow, since the carrier
> input voltage means the average level is at a couple of volts.
The second tube in the "S&S detector" is not a "CF", "CF means "Cathode
Follower", and in the "S&S detector" circuit the cathode does not follow
the grid, as it does in a true cathode follower. What you are calling the
"CF" in the "S&S detector" circuit is actually noting more than our old
friend the RCA "infinite impedance", or reflex detector. The question is
how the diode added by "S&S" acts to reduce the distortion below the level
of distortion produced by the reflex detector, if indeed it does? While
the RDH4's explanation of the reasons for the distortion reduction seem
valid as far as they go, I don't believe they are the main reason for the
potential superiority of the "S&S" detector over the RCA reflex detector.
I have a couple of ideas as to what is really going on in the circuit,
unfortunately I don't have any way of proving if my ideas are correct or
not, and this isn't a matter of building and testing the circuits, because
that only shows one detector has lower distortion than the other, it
doesn't explain why.
> There is a 150pF across the 15k, and this is charged positively by the 455 kHz
> charge
> peaks, just like a mains halfwave rectifier.
> the 15k tries to discharge the cap, so the voltage at the CF cathode is like
> the ripple voltage of a power rectifier, sawtooth shaped.
> The AF modulation causes the AF voltage to vary at the cathode, just like a
> changing
> mains voltage will give a changing DC voltage at the output of a powe
> rectifier.
>
> After the cathode detection, the S&S detected signal is fed to an output to an
> audio amp,
> via a 0.1 uF and 1M bias resistor.
Exactly, it's not really a "CF".
> That's what is shown on page 1,495 of RDH4.
>
> BUT, some 455 kHz ripple voltage will remain in this circuit, so it should be
> filtered
> out further with say an extra LPF RC filter, perhaps 50k, then another 150 pF,
>
> which would have a -3dB pole at 20 kHz, allowing a cleaner audio signal to
> get to the AF amp.
>
> The thd quoted for this detector is 0.4% at 4 Khz of AM modulation, at 80%
> modulation.
And 0.33% at 420 Hz of AM modulation, at 100% modulation.
> I think it can be improved easily, with the diode placed AFTER the CF, not
> before,
> to remove all loading effects on the IF amp, whose thd is NOT included in
> the graph for detector distortion ( we can assume ), in RDH4.
It isn't at all clear why this should make a superior detector?
> Then the CF can be a 12AT7, so in CF, its thd will be far less than a 12AU7.
> Then the idle current operation of the CF should be with say 1.5 mA flowing
> down via
> a 30k RL, or more, with the IF secondary biased up at about +40v.
>
> The diode can be a germanium type, and set up with 0.5mA of current from a
> R to some -Ve voltage, and this eliminates large variations in
> diode forward voltage variations, even at low signal levels,
> so the diode then contributes SFA thd since it spends its life turned on all
> the time.
If indeed the diode does spend "its life turned on all the time", then I
suspect that it would not make a particularly linear detector, because of
the portion of its characteristic curve that would imply it was operating
on.
Also it is my understanding that even with a biased diode of the type that
doesn't spend "its life turned on all the time", the bias must change with
signal level, or distortion the won't be minimized. This means that
either a very tight AGC circuit must be used, or the bias applied to the
diode must be made to track the signal level presented to the diode.
> The value of C charged by the diode is determined by the idle current flowing
> in the
> diode bias resistor, and should be determined experimentally.
> One of these days I'll publish my notes.
I suspect there is a bit of partisan bias in your recommendation of this
detector circuit, that is unrelated to its performance.
> Expect detector to have less than 0.1% thd at any modulation F,
> and up to 100% modulation.
I sure would like to have an AM generator that could produce a signal with
less than 0.1% distortion at 100% modulation, so I could measure these
effects.
> I wonder what Walter might say about all this.
Probably not too much, I suspect he forgot the details of the "S&S
detector" a long time ago, after all he had much more momentous things to
think about. I am told that Walter was the inventor of the transverse
recording scheme used in the Quadraplex video recorders that
revolutionized television broadcasting back in the 1950s.
Michael A. Terrell
>
> jby...@rcn.com (John Byrns) wrote in news:jbyrns-1307031343160001@216-80-
> 74-203.d.enteract.com:
>
> > Thanks, I will check the "oldradioparts" site, I never noticed that AES
> > had any of the J.W. Miller IF Transformers that I am interested in.
>
> On the AES site, hit Parts then Coils and coil forms.
>
> The Miller NOS coils are listed near the bottom of the page. Several IF
> transformers, though hard to tell if they are tube or transistor types.
I have a 1979 J.W.Miller catalog, if you need something looked up.
--
Michael A. Terrell
Central Florida